diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,6 @@
+# Changelog
+
+## 0.1.0.0 — unreleased
+
+- Initial calendar topology, grid, blank planner, paper geometry, 2027 fixtures,
+  and validation APIs.
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,14 @@
+# Contributing
+
+Please open a GitHub issue before proposing a substantial API change. Keep
+calendar arithmetic pure and deterministic, preserve explicit error results,
+and add regression coverage for every behavior change.
+
+Before submitting a change, run:
+
+```sh
+cabal check
+cabal build all
+cabal test all
+cabal haddock all
+```
diff --git a/LICENSE b/LICENSE
new file mode 100644
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Mateo Pedersen
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
--- /dev/null
+++ b/README.md
@@ -0,0 +1,132 @@
+# BetaCalendars Calendar Layout Algebra
+
+`betacalendars-calendar-layout` is a pure Haskell library for deterministic
+Gregorian month topology, seven week origins, natural and fixed six-week grids,
+undated planning grids, and physical print-layout measurements. It is useful
+without any BetaCalendars service and performs no network access.
+
+## Install
+
+Add the package to a Cabal project:
+
+```cabal
+build-depends: betacalendars-calendar-layout
+```
+
+The runtime dependency surface is intentionally small: `base` and `time`.
+
+## Quick start
+
+```haskell
+import BetaCalendars.CalendarLayout
+
+januaryTopology :: MonthTopology
+januaryTopology = monthTopology (Year 2027) January MondayStart
+
+sundayJanuary :: MonthGrid
+sundayJanuary = buildMonthGrid
+  (Year 2027) January SundayStart FixedSixWeeks IncludeAdjacentDates
+```
+
+`NaturalRows` uses the actual four-to-six row month shape. `FixedSixWeeks`
+always contains exactly 42 cells. Set `BlankAdjacentCells` to keep adjacent
+dates out of the visible grid while retaining explicit `EmptyCell` semantics.
+
+## Examples
+
+January 2027 topology:
+
+```haskell
+monthTopology (Year 2027) January MondayStart
+```
+
+Compare week origins:
+
+```haskell
+monthTopology (Year 2027) January MondayStart
+monthTopology (Year 2027) January SundayStart
+```
+
+Fixed 42-cell grid:
+
+```haskell
+length (gridCells (buildMonthGrid (Year 2027) January SundayStart
+  FixedSixWeeks IncludeAdjacentDates)) == 42
+```
+
+A4 portrait metrics and an undated 6 × 7 planner:
+
+```haskell
+import BetaCalendars.CalendarLayout.Blank
+import BetaCalendars.CalendarLayout.Paper
+
+layoutMetrics (defaultLayoutSpec A4 Portrait 6)
+blankGrid SixWeeks
+```
+
+Runnable versions of these examples are in `examples/`.
+
+## Computed 2027 topology
+
+The table is derived from the Gregorian calendar model. Weekday labels and row
+counts are computed, not stored as hand-entered month data. “Printable
+reference” links point to human-readable Beta Calendars pages.
+
+| Month | Days | First weekday | Last weekday | Monday-first rows | Sunday-first rows | Printable reference |
+|---|---:|---|---|---:|---:|
+| January | 31 | Friday | Sunday | 5 | 6 | [January Calendar](https://www.betacalendars.com/january-calendar.html) |
+| February | 28 | Monday | Sunday | 4 | 5 | [February Calendar](https://www.betacalendars.com/february-calendar.html) |
+| March | 31 | Monday | Wednesday | 5 | 5 | [March Calendar](https://www.betacalendars.com/march-calendar.html) |
+| April | 30 | Thursday | Friday | 5 | 5 | [April Calendar](https://www.betacalendars.com/april-calendar.html) |
+| May | 31 | Saturday | Monday | 6 | 6 | [May Calendar](https://www.betacalendars.com/may-calendar.html) |
+| June | 30 | Tuesday | Wednesday | 5 | 5 | [June Calendar](https://www.betacalendars.com/june-calendar.html) |
+| July | 31 | Thursday | Saturday | 5 | 5 | [July Calendar](https://www.betacalendars.com/july-calendar.html) |
+| August | 31 | Sunday | Tuesday | 6 | 5 | [August Calendar](https://www.betacalendars.com/august-calendar.html) |
+| September | 30 | Wednesday | Thursday | 5 | 5 | [September Calendar](https://www.betacalendars.com/september-calendar.html) |
+| October | 31 | Friday | Sunday | 5 | 6 | [October Calendar](https://www.betacalendars.com/october-calendar.html) |
+| November | 30 | Monday | Tuesday | 5 | 5 | [November Calendar](https://www.betacalendars.com/november-calendar.html) |
+| December | 31 | Wednesday | Friday | 5 | 5 | [December Calendar](https://www.betacalendars.com/december-calendar.html) |
+
+## Human-readable 2027 calendar references
+
+The library calculates calendar topology independently from Gregorian civil
+rules. The following Beta Calendars pages are human-readable printable
+references for visual comparison with the computed month structures.
+
+- [Beta Calendars](https://www.betacalendars.com/)
+- [Blank Calendar](https://www.betacalendars.com/blank-calendar)
+- [January Calendar](https://www.betacalendars.com/january-calendar.html)
+- [February Calendar](https://www.betacalendars.com/february-calendar.html)
+- [March Calendar](https://www.betacalendars.com/march-calendar.html)
+- [April Calendar](https://www.betacalendars.com/april-calendar.html)
+- [May Calendar](https://www.betacalendars.com/may-calendar.html)
+- [June Calendar](https://www.betacalendars.com/june-calendar.html)
+- [July Calendar](https://www.betacalendars.com/july-calendar.html)
+- [August Calendar](https://www.betacalendars.com/august-calendar.html)
+- [September Calendar](https://www.betacalendars.com/september-calendar.html)
+- [October Calendar](https://www.betacalendars.com/october-calendar.html)
+- [November Calendar](https://www.betacalendars.com/november-calendar.html)
+- [December Calendar](https://www.betacalendars.com/december-calendar.html)
+
+## Modules
+
+- `BetaCalendars.CalendarLayout`: curated high-level API.
+- `.Civil`: Gregorian year, month, date and weekday operations.
+- `.WeekStart`: week-origin rotation and offsets.
+- `.Grid`: natural/fixed month grids and topology signatures.
+- `.Paper`: millimetres, paper sizes, margins and layout metrics.
+- `.Blank`: undated 5 × 7 and 6 × 7 planning grids.
+- `.Year2027`: computed January–December 2027 fixtures.
+- `.Boundary2027`: November 2026 through February 2027 fixtures.
+- `.Validation`: structured validation diagnostics and the 1900–2100 matrix.
+
+## Validation
+
+Run `cabal check`, `cabal build all`, `cabal test all`, and `cabal haddock all`.
+The deterministic test suite checks every month from 1900 through 2100 for all
+seven week starts and both grid modes, along with leap-year boundaries,
+paper-size/orientation combinations, blank grids, and the 2026–2027 boundary.
+
+## License
+
+MIT. See [LICENSE](LICENSE).
diff --git a/SECURITY.md b/SECURITY.md
new file mode 100644
--- /dev/null
+++ b/SECURITY.md
@@ -0,0 +1,6 @@
+# Security policy
+
+This library performs local, pure calendar and physical-layout calculations;
+it does not make network requests or process untrusted files. Please report
+potential security issues privately to the maintainer email listed in the
+Cabal metadata rather than publishing exploit details in an issue.
diff --git a/betacalendars-calendar-layout.cabal b/betacalendars-calendar-layout.cabal
new file mode 100644
--- /dev/null
+++ b/betacalendars-calendar-layout.cabal
@@ -0,0 +1,85 @@
+cabal-version:      2.4
+name:               betacalendars-calendar-layout
+version:            0.1.0.0
+synopsis:           Pure calendar-grid topology and physical print-layout algebra
+description:
+  A deterministic Haskell library for Gregorian month topology, all seven
+  week origins, natural and fixed six-week grids, undated planning grids,
+  paper geometry, and structured validation.
+category:           Data, Time
+license:            MIT
+license-file:       LICENSE
+author:             Mateo Pedersen
+maintainer:         betamateopedersen@gmail.com
+homepage:           https://www.betacalendars.com/
+bug-reports:        https://github.com/mateopedersen/haskell-betacalendars-calendar-layout/issues
+build-type:         Simple
+extra-doc-files:    README.md CHANGELOG.md CONTRIBUTING.md SECURITY.md
+extra-source-files: cabal.project
+                    examples/Basic.hs
+                    examples/Paper.hs
+                    examples/Blank.hs
+
+library
+  hs-source-dirs:      src
+  exposed-modules:
+      BetaCalendars.CalendarLayout
+      BetaCalendars.CalendarLayout.Civil
+      BetaCalendars.CalendarLayout.WeekStart
+      BetaCalendars.CalendarLayout.Grid
+      BetaCalendars.CalendarLayout.Paper
+      BetaCalendars.CalendarLayout.Blank
+      BetaCalendars.CalendarLayout.Year2027
+      BetaCalendars.CalendarLayout.Boundary2027
+      BetaCalendars.CalendarLayout.Validation
+  build-depends:
+      base >=4.16 && <5
+    , time >=1.11 && <2
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+
+test-suite calendar-layout-tests
+  type:                exitcode-stdio-1.0
+  hs-source-dirs:      test
+  main-is:             Main.hs
+  build-depends:
+      base >=4.16 && <5
+    , betacalendars-calendar-layout
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+
+executable example-basic
+  hs-source-dirs:      examples
+  main-is:             Basic.hs
+  build-depends:
+      base >=4.16 && <5
+    , betacalendars-calendar-layout
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+
+executable example-paper
+  hs-source-dirs:      examples
+  main-is:             Paper.hs
+  build-depends:
+      base >=4.16 && <5
+    , betacalendars-calendar-layout
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+
+executable example-blank
+  hs-source-dirs:      examples
+  main-is:             Blank.hs
+  build-depends:
+      base >=4.16 && <5
+    , betacalendars-calendar-layout
+  default-language:    Haskell2010
+  ghc-options:         -Wall
+
+source-repository head
+  type:                git
+  location:            https://github.com/mateopedersen/haskell-betacalendars-calendar-layout
+
+source-repository this
+  type:                git
+  location:            https://github.com/mateopedersen/haskell-betacalendars-calendar-layout
+  tag:                 v0.1.0.0
diff --git a/cabal.project b/cabal.project
new file mode 100644
--- /dev/null
+++ b/cabal.project
@@ -0,0 +1,1 @@
+packages: .
diff --git a/examples/Basic.hs b/examples/Basic.hs
new file mode 100644
--- /dev/null
+++ b/examples/Basic.hs
@@ -0,0 +1,10 @@
+module Main (main) where
+
+import BetaCalendars.CalendarLayout
+import BetaCalendars.CalendarLayout.Year2027 (year2027Topologies)
+
+main :: IO ()
+main = do
+  print (monthTopology (Year 2027) January MondayStart)
+  print (length (gridCells (buildMonthGrid (Year 2027) January SundayStart FixedSixWeeks IncludeAdjacentDates)))
+  print (length year2027Topologies)
diff --git a/examples/Blank.hs b/examples/Blank.hs
new file mode 100644
--- /dev/null
+++ b/examples/Blank.hs
@@ -0,0 +1,9 @@
+module Main (main) where
+
+import BetaCalendars.CalendarLayout.Blank
+import BetaCalendars.CalendarLayout.Paper
+
+main :: IO ()
+main = do
+  print (length (blankCells (blankGrid SixWeeks)))
+  print (blankLayout A4 Portrait SixWeeks)
diff --git a/examples/Paper.hs b/examples/Paper.hs
new file mode 100644
--- /dev/null
+++ b/examples/Paper.hs
@@ -0,0 +1,6 @@
+module Main (main) where
+
+import BetaCalendars.CalendarLayout.Paper
+
+main :: IO ()
+main = print (layoutMetrics (defaultLayoutSpec A4 Portrait 6))
diff --git a/src/BetaCalendars/CalendarLayout.hs b/src/BetaCalendars/CalendarLayout.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout.hs
@@ -0,0 +1,36 @@
+-- | High-level API for deterministic civil calendar grids and print geometry.
+-- The project homepage is <https://www.betacalendars.com/>.
+module BetaCalendars.CalendarLayout
+  ( Year (..)
+  , Month (..)
+  , allMonths
+  , CivilDate
+  , civilDate
+  , dateYear
+  , dateMonth
+  , dateDay
+  , Weekday (..)
+  , WeekStart (..)
+  , allWeekStarts
+  , GridMode (..)
+  , AdjacentPolicy (..)
+  , CellRelation (..)
+  , GridCell (..)
+  , MonthGrid (..)
+  , MonthTopology (..)
+  , buildMonthGrid
+  , monthTopology
+  , Millimeters (..)
+  , PaperSize (..)
+  , Orientation (..)
+  , Margins (..)
+  , LayoutSpec (..)
+  , LayoutError (..)
+  , LayoutMetrics (..)
+  , layoutMetrics
+  ) where
+
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.Grid
+import BetaCalendars.CalendarLayout.Paper
+import BetaCalendars.CalendarLayout.WeekStart
diff --git a/src/BetaCalendars/CalendarLayout/Blank.hs b/src/BetaCalendars/CalendarLayout/Blank.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Blank.hs
@@ -0,0 +1,50 @@
+-- | Undated planning grids; blank cells deliberately carry no civil dates.
+-- See the [printable blank calendar](https://www.betacalendars.com/blank-calendar).
+module BetaCalendars.CalendarLayout.Blank
+  ( BlankRows (..)
+  , BlankCell (..)
+  , BlankGrid (..)
+  , blankGrid
+  , blankLayout
+  , blankWritingArea
+  , compareBlankLayouts
+  ) where
+
+import BetaCalendars.CalendarLayout.Paper
+
+-- | Supported undated planner heights.
+data BlankRows = FiveWeeks | SixWeeks
+  deriving (Eq, Ord, Show, Read)
+
+-- | A blank position with zero-based row and column coordinates.
+data BlankCell = BlankCell
+  { blankRow :: Int -- ^ Row, starting at zero.
+  , blankColumn :: Int -- ^ Column, starting at zero.
+  } deriving (Eq, Ord, Show, Read)
+
+-- | A date-free rectangular planner grid.
+data BlankGrid = BlankGrid
+  { blankRows :: BlankRows -- ^ Five or six week rows.
+  , blankColumns :: Int -- ^ Seven weekday columns.
+  , blankCells :: [BlankCell] -- ^ Cells in row-major order.
+  } deriving (Eq, Show)
+
+-- | Create an undated grid with seven columns and the requested row count.
+blankGrid :: BlankRows -> BlankGrid
+blankGrid rows = BlankGrid rows 7 [BlankCell r c | r <- [0 .. rowCount - 1], c <- [0 .. 6]]
+  where rowCount = case rows of FiveWeeks -> 5; SixWeeks -> 6
+
+-- | Compute paper metrics for an undated grid.
+blankLayout :: PaperSize -> Orientation -> BlankRows -> Either LayoutError LayoutMetrics
+blankLayout paper orientation rows = layoutMetrics (defaultLayoutSpec paper orientation rowCount)
+  where rowCount = case rows of FiveWeeks -> 5; SixWeeks -> 6
+
+-- | Usable writing area in square millimetres.
+blankWritingArea :: LayoutMetrics -> Rational
+blankWritingArea = writingArea
+
+-- | Signed ratio of usable writing areas; zero is returned for a zero baseline.
+compareBlankLayouts :: LayoutMetrics -> LayoutMetrics -> Rational
+compareBlankLayouts a b
+  | writingArea b == 0 = 0
+  | otherwise = writingArea a / writingArea b
diff --git a/src/BetaCalendars/CalendarLayout/Boundary2027.hs b/src/BetaCalendars/CalendarLayout/Boundary2027.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Boundary2027.hs
@@ -0,0 +1,27 @@
+-- | Computed regression fixtures spanning the 2026–2027 year boundary.
+-- Printable comparisons: [November](https://www.betacalendars.com/november-calendar.html),
+-- [December](https://www.betacalendars.com/december-calendar.html),
+-- [January](https://www.betacalendars.com/january-calendar.html), and
+-- [February](https://www.betacalendars.com/february-calendar.html).
+module BetaCalendars.CalendarLayout.Boundary2027
+  ( boundary2027Months
+  , boundary2027Grids
+  ) where
+
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.Grid
+import BetaCalendars.CalendarLayout.WeekStart
+
+-- | November and December 2026 followed by January and February 2027.
+boundary2027Months :: [(Year, Month)]
+boundary2027Months =
+  [ (Year 2026, November)
+  , (Year 2026, December)
+  , (Year 2027, January)
+  , (Year 2027, February)
+  ]
+
+-- | Monday-first natural month grids for 'boundary2027Months'.
+boundary2027Grids :: [MonthGrid]
+boundary2027Grids =
+  [buildMonthGrid y m MondayStart NaturalRows IncludeAdjacentDates | (y, m) <- boundary2027Months]
diff --git a/src/BetaCalendars/CalendarLayout/Civil.hs b/src/BetaCalendars/CalendarLayout/Civil.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Civil.hs
@@ -0,0 +1,141 @@
+-- | Proleptic Gregorian civil dates and month boundaries. No time zone or
+-- timestamp semantics are involved.
+module BetaCalendars.CalendarLayout.Civil
+  ( Year (..)
+  , Month (..)
+  , allMonths
+  , CivilDate
+  , civilDate
+  , civilDateDay
+  , dateYear
+  , dateMonth
+  , dateDay
+  , Weekday (..)
+  , weekdayOf
+  , isLeapYear
+  , daysInMonth
+  , firstDate
+  , lastDate
+  , firstWeekday
+  , lastWeekday
+  , nextCivilDate
+  , previousCivilDate
+  ) where
+
+import Data.Time.Calendar (Day, addDays, dayOfWeek, fromGregorian, fromGregorianValid, toGregorian)
+import qualified Data.Time.Calendar.WeekDate as W
+
+-- | A Gregorian calendar year. 'Integer' avoids an artificial machine-year limit.
+newtype Year = Year
+  { unYear :: Integer -- ^ Numeric Gregorian year.
+  }
+  deriving (Eq, Ord, Show)
+
+-- | The twelve Gregorian months, in calendar order. The constructor order is
+-- January through December and is used by 'allMonths'.
+data Month = January | February | March | April | May | June
+  | July | August | September | October | November | December
+  deriving (Eq, Ord, Enum, Bounded, Show, Read)
+
+-- | Months in chronological order from January to December.
+allMonths :: [Month]
+allMonths = [minBound .. maxBound]
+
+-- | A valid Gregorian civil date, represented internally by @time@'s 'Day'.
+newtype CivilDate = CivilDate
+  { civilDateDay :: Day -- ^ Underlying day value.
+  }
+  deriving (Eq, Ord, Show)
+
+-- | Construct a date, returning 'Nothing' for an invalid month day.
+civilDate :: Year -> Month -> Int -> Maybe CivilDate
+civilDate (Year y) m d = CivilDate <$> fromGregorianValid y (monthNumber m) d
+
+-- | Year component of a valid civil date.
+dateYear :: CivilDate -> Year
+dateYear (CivilDate d) = let (y, _, _) = toGregorian d in Year y
+
+-- | Month component of a valid civil date.
+dateMonth :: CivilDate -> Month
+dateMonth (CivilDate d) = let (_, m, _) = toGregorian d in monthFromNumber m
+
+-- | Day-of-month component, in the range 1–31.
+dateDay :: CivilDate -> Int
+dateDay (CivilDate d) = let (_, _, day) = toGregorian d in day
+
+-- | Monday-based weekday names, ordered Monday through Sunday.
+data Weekday = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
+  deriving (Eq, Ord, Enum, Bounded, Show, Read)
+
+-- | Weekday of a valid civil date.
+weekdayOf :: CivilDate -> Weekday
+weekdayOf (CivilDate d) = case dayOfWeek d of
+  W.Monday -> Monday
+  W.Tuesday -> Tuesday
+  W.Wednesday -> Wednesday
+  W.Thursday -> Thursday
+  W.Friday -> Friday
+  W.Saturday -> Saturday
+  W.Sunday -> Sunday
+
+-- | Gregorian leap-year rule, including century exceptions.
+isLeapYear :: Year -> Bool
+isLeapYear (Year y) = y `mod` 4 == 0 && (y `mod` 100 /= 0 || y `mod` 400 == 0)
+
+-- | Number of days in a month.
+daysInMonth :: Year -> Month -> Int
+daysInMonth y m = case m of
+  January -> 31
+  February -> if isLeapYear y then 29 else 28
+  March -> 31
+  April -> 30
+  May -> 31
+  June -> 30
+  July -> 31
+  August -> 31
+  September -> 30
+  October -> 31
+  November -> 30
+  December -> 31
+
+-- | First civil date in a month.
+firstDate :: Year -> Month -> CivilDate
+firstDate y m = CivilDate (fromGregorian (unYear y) (monthNumber m) 1)
+
+-- | Last civil date in a month.
+lastDate :: Year -> Month -> CivilDate
+lastDate y m = CivilDate (fromGregorian (unYear y) (monthNumber m) (daysInMonth y m))
+
+-- | Weekday of the first date in a month.
+firstWeekday :: Year -> Month -> Weekday
+firstWeekday year month = weekdayOf (firstDate year month)
+
+-- | Weekday of the last date in a month.
+lastWeekday :: Year -> Month -> Weekday
+lastWeekday year month = weekdayOf (lastDate year month)
+
+-- | Following civil day.
+nextCivilDate :: CivilDate -> CivilDate
+nextCivilDate (CivilDate d) = CivilDate (addDays 1 d)
+
+-- | Previous civil day.
+previousCivilDate :: CivilDate -> CivilDate
+previousCivilDate (CivilDate d) = CivilDate (addDays (-1) d)
+
+monthNumber :: Month -> Int
+monthNumber = (+ 1) . fromEnum
+
+monthFromNumber :: Int -> Month
+monthFromNumber n = case n of
+  1 -> January
+  2 -> February
+  3 -> March
+  4 -> April
+  5 -> May
+  6 -> June
+  7 -> July
+  8 -> August
+  9 -> September
+  10 -> October
+  11 -> November
+  _ -> December
diff --git a/src/BetaCalendars/CalendarLayout/Grid.hs b/src/BetaCalendars/CalendarLayout/Grid.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Grid.hs
@@ -0,0 +1,155 @@
+-- | Pure month-grid construction and descriptive topology values.
+module BetaCalendars.CalendarLayout.Grid
+  ( GridMode (..)
+  , AdjacentPolicy (..)
+  , CellRelation (..)
+  , GridCell (..)
+  , MonthGrid (..)
+  , MonthTopology (..)
+  , TopologySignature (..)
+  , buildMonthGrid
+  , monthTopology
+  , naturalRowCount
+  , leadingCellCount
+  , trailingCellCount
+  , gridOccupancy
+  , topologySignature
+  ) where
+
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.WeekStart
+
+-- | Use the month’s natural shape or a fixed 6 × 7 page grid.
+data GridMode = NaturalRows | FixedSixWeeks
+  deriving (Eq, Ord, Show, Read)
+
+-- | Whether out-of-month grid positions show dates or remain undated blanks.
+data AdjacentPolicy = IncludeAdjacentDates | BlankAdjacentCells
+  deriving (Eq, Ord, Show, Read)
+
+-- | Semantic relation of a cell to the requested month.
+data CellRelation = PreviousMonth | CurrentMonth | NextMonth | EmptyCell
+  deriving (Eq, Ord, Show, Read)
+
+-- | One month-grid position and its date, if one is shown.
+data GridCell = GridCell
+  { cellDate :: Maybe CivilDate -- ^ No date for an intentionally blank cell.
+  , cellRelation :: CellRelation -- ^ Position’s semantic relation.
+  } deriving (Eq, Show)
+
+-- | A row-major rectangular month grid with its construction parameters.
+data MonthGrid = MonthGrid
+  { gridYear :: Year -- ^ Requested civil year.
+  , gridMonth :: Month -- ^ Requested month.
+  , gridWeekStart :: WeekStart -- ^ First weekday in each row.
+  , gridMode :: GridMode -- ^ Natural or fixed six-week shape.
+  , gridAdjacentPolicy :: AdjacentPolicy -- ^ Visibility of adjacent dates.
+  , gridRows :: Int -- ^ Number of rows in the result.
+  , gridCells :: [GridCell] -- ^ Cells in row-major order.
+  } deriving (Eq, Show)
+
+-- | Week-origin-specific structure of a Gregorian month.
+data MonthTopology = MonthTopology
+  { topologyDays :: Int -- ^ Number of dates in the month.
+  , topologyFirstWeekday :: Weekday -- ^ Weekday of its first date.
+  , topologyLastWeekday :: Weekday -- ^ Weekday of its last date.
+  , topologyLeadingCells :: Int -- ^ Cells before the first date.
+  , topologyTrailingCells :: Int -- ^ Cells after the last date in a natural grid.
+  , topologyNaturalRows :: Int -- ^ Natural row count, from four through six.
+  , topologyFixedCells :: Int -- ^ Fixed grid size, always 42.
+  } deriving (Eq, Show)
+
+-- | Library-specific summary; not a standardized calendar metric.
+data TopologySignature = TopologySignature
+  { signatureDays :: Int -- ^ Current-month cell count.
+  , signatureStartOffset :: Int -- ^ Leading-cell count.
+  , signatureNaturalRows :: Int -- ^ Natural row count.
+  , signatureTrailingCapacity :: Int -- ^ Trailing cells in the natural grid.
+  } deriving (Eq, Ord, Show, Read)
+
+-- | Build a deterministic month grid for any week origin and adjacent policy.
+buildMonthGrid :: Year -> Month -> WeekStart -> GridMode -> AdjacentPolicy -> MonthGrid
+buildMonthGrid year month weekStart mode policy = MonthGrid
+  { gridYear = year
+  , gridMonth = month
+  , gridWeekStart = weekStart
+  , gridMode = mode
+  , gridAdjacentPolicy = policy
+  , gridRows = rows
+  , gridCells = map makeCell offsets
+  }
+  where
+    leading = leadingCellCount year month weekStart
+    count = daysInMonth year month
+    naturalCells = leading + count
+    rows = case mode of
+      NaturalRows -> (naturalCells + 6) `div` 7
+      FixedSixWeeks -> 6
+    size = rows * 7
+    first = firstDate year month
+    offsets = [-leading .. size - leading - 1]
+    makeCell n =
+      let date = shiftDate first n
+          relation = relationTo year month date
+          visibleDate = case (policy, relation) of
+            (BlankAdjacentCells, CurrentMonth) -> Just date
+            (BlankAdjacentCells, _) -> Nothing
+            (IncludeAdjacentDates, _) -> Just date
+      in GridCell visibleDate (if visibleDate == Nothing then EmptyCell else relation)
+
+-- | Describe the month’s topology for one configured week origin.
+monthTopology :: Year -> Month -> WeekStart -> MonthTopology
+monthTopology year month start = MonthTopology
+  { topologyDays = daysInMonth year month
+  , topologyFirstWeekday = firstWeekday year month
+  , topologyLastWeekday = lastWeekday year month
+  , topologyLeadingCells = leading
+  , topologyTrailingCells = naturalRows * 7 - leading - daysInMonth year month
+  , topologyNaturalRows = naturalRows
+  , topologyFixedCells = 42
+  }
+  where
+    leading = leadingCellCount year month start
+    naturalRows = naturalRowCount year month start
+
+-- | Natural row count, which is always four, five, or six.
+naturalRowCount :: Year -> Month -> WeekStart -> Int
+naturalRowCount year month start = (leadingCellCount year month start + daysInMonth year month + 6) `div` 7
+
+-- | Number of cells before the first day under the selected week origin.
+leadingCellCount :: Year -> Month -> WeekStart -> Int
+leadingCellCount year month start = offsetFromWeekStart start (firstWeekday year month)
+
+-- | Number of cells after the last day in a natural grid.
+trailingCellCount :: Year -> Month -> WeekStart -> Int
+trailingCellCount year month start = naturalRowCount year month start * 7 - leadingCellCount year month start - daysInMonth year month
+
+-- | Fraction of the grid occupied by current-month dates; empty grids yield 0.
+gridOccupancy :: MonthGrid -> Rational
+gridOccupancy grid
+  | null (gridCells grid) = 0
+  | otherwise = fromIntegral current / fromIntegral (length (gridCells grid))
+  where current = length (filter ((== CurrentMonth) . cellRelation) (gridCells grid))
+
+-- | Convert a topology to this library’s descriptive signature value.
+topologySignature :: MonthTopology -> TopologySignature
+topologySignature t = TopologySignature
+  (topologyDays t) (topologyLeadingCells t) (topologyNaturalRows t) (topologyTrailingCells t)
+
+shiftDate :: CivilDate -> Int -> CivilDate
+shiftDate date n = case civilDate (dateYear date) (dateMonth date) (dateDay date + n) of
+  Just result -> result
+  Nothing -> shiftAcrossMonth date n
+
+-- Calendar arithmetic through adjacent month boundaries without partial functions.
+shiftAcrossMonth :: CivilDate -> Int -> CivilDate
+shiftAcrossMonth date n
+  | n < 0 = shiftAcrossMonth (previousCivilDate date) (n + 1)
+  | n > 0 = shiftAcrossMonth (nextCivilDate date) (n - 1)
+  | otherwise = date
+
+relationTo :: Year -> Month -> CivilDate -> CellRelation
+relationTo y m date
+  | dateYear date == y && dateMonth date == m = CurrentMonth
+  | date < firstDate y m = PreviousMonth
+  | otherwise = NextMonth
diff --git a/src/BetaCalendars/CalendarLayout/Paper.hs b/src/BetaCalendars/CalendarLayout/Paper.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Paper.hs
@@ -0,0 +1,151 @@
+-- | Physical paper dimensions and deterministic printable-grid geometry.
+-- Printable comparisons: [blank](https://www.betacalendars.com/blank-calendar),
+-- [January](https://www.betacalendars.com/january-calendar.html),
+-- [July](https://www.betacalendars.com/july-calendar.html), and
+-- [December](https://www.betacalendars.com/december-calendar.html).
+module BetaCalendars.CalendarLayout.Paper
+  ( Millimeters (..)
+  , PaperSize (..)
+  , Orientation (..)
+  , Margins (..)
+  , LayoutSpec (..)
+  , LayoutError (..)
+  , LayoutMetrics (..)
+  , paperDimensions
+  , layoutMetrics
+  , defaultLayoutSpec
+  ) where
+
+-- | Exact physical distance in millimetres, represented as a rational number.
+newtype Millimeters = Millimeters
+  { unMillimeters :: Rational -- ^ Exact distance; this is not a CSS pixel value.
+  }
+  deriving (Eq, Ord, Show, Read)
+
+-- | Standard page sizes or a custom width and height.
+data PaperSize
+  = A4 -- ^ ISO 216 A4, 210 × 297 mm.
+  | A5 -- ^ ISO 216 A5, 148 × 210 mm.
+  | Letter -- ^ US Letter, 215.9 × 279.4 mm.
+  | Legal -- ^ US Legal, 215.9 × 355.6 mm.
+  | CustomPaper Millimeters Millimeters -- ^ Custom width followed by height.
+  deriving (Eq, Ord, Show, Read)
+
+-- | Page orientation; landscape swaps the width and height.
+data Orientation = Portrait | Landscape
+  deriving (Eq, Ord, Show, Read)
+
+-- | Distances between printable content and the four page edges.
+data Margins = Margins
+  { marginTop :: Millimeters -- ^ Top edge margin.
+  , marginRight :: Millimeters -- ^ Right edge margin.
+  , marginBottom :: Millimeters -- ^ Bottom edge margin.
+  , marginLeft :: Millimeters -- ^ Left edge margin.
+  } deriving (Eq, Ord, Show, Read)
+
+-- | Inputs defining a rectangular grid and reserved title/header/notes bands.
+data LayoutSpec = LayoutSpec
+  { layoutPaper :: PaperSize -- ^ Physical sheet.
+  , layoutOrientation :: Orientation -- ^ Portrait or landscape.
+  , layoutMargins :: Margins -- ^ Edge clearances.
+  , layoutTitleHeight :: Millimeters -- ^ Height reserved for a title.
+  , layoutWeekdayHeaderHeight :: Millimeters -- ^ Height reserved for weekday labels.
+  , layoutNotesHeight :: Millimeters -- ^ Height reserved for notes.
+  , layoutRows :: Int -- ^ Positive number of grid rows.
+  , layoutColumns :: Int -- ^ Positive number of grid columns.
+  } deriving (Eq, Show, Read)
+
+-- | Reasons a proposed paper layout cannot produce positive cell geometry.
+data LayoutError
+  = InvalidPaperDimensions -- ^ Page width or height is not positive.
+  | InvalidMargins -- ^ At least one edge margin is negative.
+  | InvalidGridDimensions -- ^ Row or column count is not positive.
+  | InvalidReservedArea -- ^ A reserved band has negative height.
+  | ReservedAreaOverflow -- ^ Reserved bands leave no room for the grid.
+  | NonPositivePrintableArea -- ^ Margins leave no positive printable rectangle.
+  | NonPositiveCellGeometry -- ^ Computed cell width or height is not positive.
+  deriving (Eq, Ord, Show, Read)
+
+-- | Derived page, printable region, grid, and cell measurements.
+data LayoutMetrics = LayoutMetrics
+  { pageWidth :: Millimeters -- ^ Oriented page width.
+  , pageHeight :: Millimeters -- ^ Oriented page height.
+  , printableWidth :: Millimeters -- ^ Width after left/right margins.
+  , printableHeight :: Millimeters -- ^ Height after top/bottom margins.
+  , gridWidth :: Millimeters -- ^ Width occupied by the grid.
+  , gridHeight :: Millimeters -- ^ Height occupied by the grid.
+  , cellWidth :: Millimeters -- ^ Width of each equal grid column.
+  , cellHeight :: Millimeters -- ^ Height of each equal grid row.
+  , cellArea :: Rational -- ^ Area of one cell in square millimetres.
+  , writingArea :: Rational -- ^ Grid area in square millimetres.
+  } deriving (Eq, Show, Read)
+
+-- | Exact dimensions for standard and custom paper sizes, before orientation.
+paperDimensions :: PaperSize -> (Millimeters, Millimeters)
+paperDimensions size = case size of
+  A4 -> (mm 210, mm 297)
+  A5 -> (mm 148, mm 210)
+  Letter -> (mmFraction 2159 10, mmFraction 2794 10)
+  Legal -> (mmFraction 2159 10, mmFraction 3556 10)
+  CustomPaper w h -> (w, h)
+
+-- | A useful default with 12 mm margins, 18 mm title, 8 mm weekday band,
+-- and 20 mm notes band, using seven columns.
+defaultLayoutSpec :: PaperSize -> Orientation -> Int -> LayoutSpec
+defaultLayoutSpec paper orientation rows = LayoutSpec
+  { layoutPaper = paper
+  , layoutOrientation = orientation
+  , layoutMargins = Margins (mm 12) (mm 12) (mm 12) (mm 12)
+  , layoutTitleHeight = mm 18
+  , layoutWeekdayHeaderHeight = mm 8
+  , layoutNotesHeight = mm 20
+  , layoutRows = rows
+  , layoutColumns = 7
+  }
+
+-- | Validate a layout and compute exact dimensions, or return a domain error.
+layoutMetrics :: LayoutSpec -> Either LayoutError LayoutMetrics
+layoutMetrics spec
+  | width <= 0 || height <= 0 = Left InvalidPaperDimensions
+  | any (< 0) margins = Left InvalidMargins
+  | layoutRows spec <= 0 || layoutColumns spec <= 0 = Left InvalidGridDimensions
+  | any (< 0) reserved = Left InvalidReservedArea
+  | printableW <= 0 || printableH <= 0 = Left NonPositivePrintableArea
+  | gridH <= 0 = Left ReservedAreaOverflow
+  | cellW <= 0 || cellH <= 0 = Left NonPositiveCellGeometry
+  | otherwise = Right LayoutMetrics
+      { pageWidth = Millimeters pageW
+      , pageHeight = Millimeters pageH
+      , printableWidth = Millimeters printableW
+      , printableHeight = Millimeters printableH
+      , gridWidth = Millimeters printableW
+      , gridHeight = Millimeters gridH
+      , cellWidth = Millimeters cellW
+      , cellHeight = Millimeters cellH
+      , cellArea = cellW * cellH
+      , writingArea = printableW * gridH
+      }
+  where
+    (paperW, paperH) = paperDimensions (layoutPaper spec)
+    (width, height) = case layoutOrientation spec of
+      Portrait -> (unMillimeters paperW, unMillimeters paperH)
+      Landscape -> (unMillimeters paperH, unMillimeters paperW)
+    (top, right, bottom, left) = toTuple (layoutMargins spec)
+    pageW = width
+    pageH = height
+    margins = [top, right, bottom, left]
+    printableW = width - right - left
+    printableH = height - top - bottom
+    reserved = [unMillimeters (layoutTitleHeight spec), unMillimeters (layoutWeekdayHeaderHeight spec), unMillimeters (layoutNotesHeight spec)]
+    gridH = printableH - sum reserved
+    cellW = printableW / fromIntegral (layoutColumns spec)
+    cellH = gridH / fromIntegral (layoutRows spec)
+
+mm :: Integer -> Millimeters
+mm = Millimeters . fromInteger
+
+mmFraction :: Integer -> Integer -> Millimeters
+mmFraction n d = Millimeters (fromInteger n / fromInteger d)
+
+toTuple :: Margins -> (Rational, Rational, Rational, Rational)
+toTuple m = (unMillimeters (marginTop m), unMillimeters (marginRight m), unMillimeters (marginBottom m), unMillimeters (marginLeft m))
diff --git a/src/BetaCalendars/CalendarLayout/Validation.hs b/src/BetaCalendars/CalendarLayout/Validation.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Validation.hs
@@ -0,0 +1,79 @@
+-- | Diagnostic validators for grids, finite date ranges and paper geometry.
+module BetaCalendars.CalendarLayout.Validation
+  ( ValidationIssue (..)
+  , validateMonthGrid
+  , validateDateCompleteness
+  , validateFixedGrid
+  , validatePaperLayout
+  , validateYear
+  , validateBoundaryFixture
+  , validateFiniteRange
+  ) where
+
+import BetaCalendars.CalendarLayout.Boundary2027 (boundary2027Grids)
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.Grid
+import BetaCalendars.CalendarLayout.Paper
+import BetaCalendars.CalendarLayout.WeekStart
+
+-- | Structured failure details returned by the reusable validators.
+data ValidationIssue
+  = WrongCellCount Int Int
+  | WrongRowCount Int Int
+  | MissingCurrentDates [Int]
+  | DuplicateCurrentDates [Int]
+  | InvalidNaturalRows Int
+  | WrongBoundaryFixtureLength Int Int
+  deriving (Eq, Show)
+
+-- | Check row count, cell count, current-date completeness, and mode invariants.
+validateMonthGrid :: MonthGrid -> [ValidationIssue]
+validateMonthGrid grid = validateDateCompleteness grid ++ rowIssues ++ fixedIssues
+  where
+    expectedRows = case gridMode grid of
+      NaturalRows -> naturalRowCount (gridYear grid) (gridMonth grid) (gridWeekStart grid)
+      FixedSixWeeks -> 6
+    expectedCells = expectedRows * 7
+    rowIssues =
+      [WrongRowCount expectedRows (gridRows grid) | gridRows grid /= expectedRows]
+      ++ [WrongCellCount expectedCells (length (gridCells grid)) | length (gridCells grid) /= expectedCells]
+      ++ [InvalidNaturalRows (gridRows grid) | gridMode grid == NaturalRows && not (gridRows grid >= 4 && gridRows grid <= 6)]
+    fixedIssues = [WrongCellCount 42 (length (gridCells grid)) | gridMode grid == FixedSixWeeks && length (gridCells grid) /= 42]
+
+-- | Verify every current-month date occurs exactly once.
+validateDateCompleteness :: MonthGrid -> [ValidationIssue]
+validateDateCompleteness grid = missingIssue ++ duplicateIssue
+  where
+    observed = [dateDay d | cell <- gridCells grid, cellRelation cell == CurrentMonth, Just d <- [cellDate cell]]
+    expected = [1 .. daysInMonth (gridYear grid) (gridMonth grid)]
+    missing = filter (`notElem` observed) expected
+    duplicates = [d | d <- expected, length (filter (== d) observed) > 1]
+    missingIssue = [MissingCurrentDates missing | not (null missing)]
+    duplicateIssue = [DuplicateCurrentDates duplicates | not (null duplicates)]
+
+-- | Check the 42-cell invariant when the supplied grid is fixed mode.
+validateFixedGrid :: MonthGrid -> [ValidationIssue]
+validateFixedGrid grid
+  | gridMode grid /= FixedSixWeeks = []
+  | length (gridCells grid) == 42 && gridRows grid == 6 = validateDateCompleteness grid
+  | otherwise = [WrongCellCount 42 (length (gridCells grid))]
+
+-- | Validate paper geometry and return computed metrics on success.
+validatePaperLayout :: LayoutSpec -> Either LayoutError LayoutMetrics
+validatePaperLayout = layoutMetrics
+
+-- | Validate all months, week origins, and both grid modes for one year.
+validateYear :: Year -> [ValidationIssue]
+validateYear y = concat
+  [validateMonthGrid (buildMonthGrid y m start mode IncludeAdjacentDates)
+  | m <- allMonths, start <- allWeekStarts, mode <- [NaturalRows, FixedSixWeeks]]
+
+-- | Validate the four-month fixture spanning the 2026–2027 year boundary.
+validateBoundaryFixture :: [ValidationIssue]
+validateBoundaryFixture
+  | length boundary2027Grids == 4 = concatMap validateMonthGrid boundary2027Grids
+  | otherwise = [WrongBoundaryFixtureLength 4 (length boundary2027Grids)]
+
+-- | Validate every month from 1900 through 2100 for all seven origins and modes.
+validateFiniteRange :: [ValidationIssue]
+validateFiniteRange = concatMap validateYear [Year y | y <- [1900 .. 2100]]
diff --git a/src/BetaCalendars/CalendarLayout/WeekStart.hs b/src/BetaCalendars/CalendarLayout/WeekStart.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/WeekStart.hs
@@ -0,0 +1,46 @@
+-- | Week-origin arithmetic for all seven possible first weekdays.
+module BetaCalendars.CalendarLayout.WeekStart
+  ( WeekStart (..)
+  , allWeekStarts
+  , weekdayIndex
+  , relativeWeekdayIndex
+  , offsetFromWeekStart
+  , rotateWeekOrigin
+  ) where
+
+import BetaCalendars.CalendarLayout.Civil (Weekday (..))
+
+-- | Configurable first weekday for a displayed week.
+data WeekStart = MondayStart | TuesdayStart | WednesdayStart | ThursdayStart
+  | FridayStart | SaturdayStart | SundayStart
+  deriving (Eq, Ord, Enum, Bounded, Show, Read)
+
+-- | All seven week origins in Monday-to-Sunday order.
+allWeekStarts :: [WeekStart]
+allWeekStarts = [minBound .. maxBound]
+
+-- | Monday is 0 and Sunday is 6.
+-- | Monday-based zero-index of a weekday.
+weekdayIndex :: Weekday -> Int
+weekdayIndex = fromEnum
+
+-- | Index of a weekday in a week whose first day is the given origin.
+-- | Zero-index of a weekday relative to a selected week origin.
+relativeWeekdayIndex :: WeekStart -> Weekday -> Int
+relativeWeekdayIndex origin weekday = (weekdayIndex weekday - startIndex origin) `mod` 7
+
+-- | Number of leading cells before the first day of a month.
+-- | Leading-cell count for a date with the given weekday.
+offsetFromWeekStart :: WeekStart -> Weekday -> Int
+offsetFromWeekStart = relativeWeekdayIndex
+
+-- | Rotate weekdays so that the configured origin appears first.
+-- | Monday-to-Sunday weekdays rotated to start at the selected origin.
+rotateWeekOrigin :: WeekStart -> [Weekday]
+rotateWeekOrigin origin = take 7 (drop start (weekdays ++ weekdays))
+  where
+    weekdays = [Monday .. Sunday]
+    start = startIndex origin
+
+startIndex :: WeekStart -> Int
+startIndex = fromEnum
diff --git a/src/BetaCalendars/CalendarLayout/Year2027.hs b/src/BetaCalendars/CalendarLayout/Year2027.hs
new file mode 100644
--- /dev/null
+++ b/src/BetaCalendars/CalendarLayout/Year2027.hs
@@ -0,0 +1,47 @@
+-- | Computed structural fixtures for the 2027 calendar year.
+-- Printable comparisons: [Beta Calendars](https://www.betacalendars.com/),
+-- [January](https://www.betacalendars.com/january-calendar.html),
+-- [February](https://www.betacalendars.com/february-calendar.html),
+-- [March](https://www.betacalendars.com/march-calendar.html),
+-- [April](https://www.betacalendars.com/april-calendar.html),
+-- [May](https://www.betacalendars.com/may-calendar.html),
+-- [June](https://www.betacalendars.com/june-calendar.html),
+-- [July](https://www.betacalendars.com/july-calendar.html),
+-- [August](https://www.betacalendars.com/august-calendar.html),
+-- [September](https://www.betacalendars.com/september-calendar.html),
+-- [October](https://www.betacalendars.com/october-calendar.html),
+-- [November](https://www.betacalendars.com/november-calendar.html),
+-- [December](https://www.betacalendars.com/december-calendar.html).
+module BetaCalendars.CalendarLayout.Year2027
+  ( year2027Topologies
+  , mondayFirst2027
+  , sundayFirst2027
+  , fixedGridOccupancy2027
+  ) where
+
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.Grid
+import BetaCalendars.CalendarLayout.WeekStart
+
+-- | Monday-first computed topology for each month of 2027.
+year2027Topologies :: [(Month, MonthTopology)]
+year2027Topologies = [(m, monthTopology year2027 m MondayStart) | m <- allMonths]
+
+-- | Natural grids for every 2027 month, starting weeks on Monday.
+mondayFirst2027 :: [MonthGrid]
+mondayFirst2027 = grids MondayStart NaturalRows
+
+-- | Natural grids for every 2027 month, starting weeks on Sunday.
+sundayFirst2027 :: [MonthGrid]
+sundayFirst2027 = grids SundayStart NaturalRows
+
+-- | Current-month occupancy fractions for fixed six-week 2027 grids.
+fixedGridOccupancy2027 :: [(Month, Rational)]
+fixedGridOccupancy2027 =
+  [(m, gridOccupancy (buildMonthGrid year2027 m MondayStart FixedSixWeeks IncludeAdjacentDates)) | m <- allMonths]
+
+year2027 :: Year
+year2027 = Year 2027
+
+grids :: WeekStart -> GridMode -> [MonthGrid]
+grids start mode = [buildMonthGrid year2027 m start mode IncludeAdjacentDates | m <- allMonths]
diff --git a/test/Main.hs b/test/Main.hs
new file mode 100644
--- /dev/null
+++ b/test/Main.hs
@@ -0,0 +1,72 @@
+module Main (main) where
+
+import BetaCalendars.CalendarLayout.Blank
+import BetaCalendars.CalendarLayout.Boundary2027
+import BetaCalendars.CalendarLayout.Civil
+import BetaCalendars.CalendarLayout.Grid
+import BetaCalendars.CalendarLayout.Paper
+import BetaCalendars.CalendarLayout.Validation
+import BetaCalendars.CalendarLayout.WeekStart
+import System.Exit (exitFailure)
+
+main :: IO ()
+main = do
+  let finite =
+        [ (show y ++ "/" ++ show m ++ "/" ++ show start ++ "/" ++ show mode,
+            let grid = buildMonthGrid y m start mode IncludeAdjacentDates
+                firstCurrent = case [i | (i, cell) <- zip [0 ..] (gridCells grid), cellRelation cell == CurrentMonth] of
+                  i : _ -> i
+                  [] -> -1
+                expectedOffset = offsetFromWeekStart start (firstWeekday y m)
+            in null (validateMonthGrid grid)
+              && firstCurrent == expectedOffset
+              && (mode /= FixedSixWeeks || length (gridCells grid) == 42))
+        | y <- [Year n | n <- [1900 .. 2100]]
+        , m <- allMonths
+        , start <- allWeekStarts
+        , mode <- [NaturalRows, FixedSixWeeks]
+        ]
+      leaps =
+        [ ("1900 common", not (isLeapYear (Year 1900)))
+        , ("2000 leap", isLeapYear (Year 2000))
+        , ("2024 leap", isLeapYear (Year 2024))
+        , ("2027 common", not (isLeapYear (Year 2027)))
+        , ("2100 common", not (isLeapYear (Year 2100)))
+        , ("2400 leap", isLeapYear (Year 2400))
+        ]
+      papers =
+        [ (show paper ++ " " ++ show orientation,
+            case layoutMetrics (defaultLayoutSpec paper orientation 6) of
+              Right metrics -> unMillimeters (cellWidth metrics) > 0
+                && unMillimeters (cellHeight metrics) > 0 && cellArea metrics > 0
+              Left _ -> False)
+        | paper <- [A4, A5, Letter, Legal]
+        , orientation <- [Portrait, Landscape]
+        ]
+      blankChecks =
+        [ ("blank five weeks", length (blankCells (blankGrid FiveWeeks)) == 35)
+        , ("blank six weeks", length (blankCells (blankGrid SixWeeks)) == 42)
+        , ("blank adjacent dates", all ((== EmptyCell) . cellRelation)
+            (take 4 (gridCells (buildMonthGrid (Year 2027) January MondayStart NaturalRows BlankAdjacentCells))))
+        ]
+      boundaryChecks =
+        [ ("Nov 2026 has 30 days", daysInMonth (Year 2026) November == 30)
+        , ("Dec 2026 has 31 days", daysInMonth (Year 2026) December == 31)
+        , ("Feb 2027 has 28 days", daysInMonth (Year 2027) February == 28)
+        , ("year transition", dateYear (nextCivilDate (lastDate (Year 2026) December)) == Year 2027)
+        , ("boundary fixture validation", null validateBoundaryFixture && length boundary2027Grids == 4)
+        ]
+      invalidLayout = (defaultLayoutSpec A4 Portrait 6)
+        { layoutMargins = Margins (Millimeters (-1)) (Millimeters 12) (Millimeters 12) (Millimeters 12) }
+      overflowLayout = (defaultLayoutSpec A4 Portrait 6) { layoutNotesHeight = Millimeters 1000 }
+      invalidChecks =
+        [ ("negative margin rejected", case layoutMetrics invalidLayout of Left InvalidMargins -> True; _ -> False)
+        , ("reserved-area overflow rejected", case layoutMetrics overflowLayout of Left ReservedAreaOverflow -> True; _ -> False)
+        ]
+      assertions = leaps ++ papers ++ blankChecks ++ boundaryChecks ++ invalidChecks ++ finite
+      failures = filter (not . snd) assertions
+  if null failures
+    then putStrLn ("PASS: " ++ show (length assertions) ++ " deterministic assertions")
+    else do
+      mapM_ (putStrLn . ("FAIL: " ++) . fst) failures
+      exitFailure