diff --git a/CHANGELOG.md b/CHANGELOG.md
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--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,5 @@
+# Revision history for primitive-element
+
+## 0.1.0.0 -- YYYY-mm-dd
+
+* First version. Released on an unsuspecting world.
diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,30 @@
+Copyright (c) 2019, Andrew Martin
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+
+    * Neither the name of Andrew Martin nor the names of other
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/Setup.hs b/Setup.hs
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--- /dev/null
+++ b/Setup.hs
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+import Distribution.Simple
+main = defaultMain
diff --git a/primitive-offset.cabal b/primitive-offset.cabal
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--- /dev/null
+++ b/primitive-offset.cabal
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+cabal-version: 2.2
+name: primitive-offset
+version: 0.1.0.0
+synopsis: Types for offsets into unboxed arrays
+homepage: https://github.com/andrewthad/primitive-offset
+bug-reports: https://github.com/andrewthad/primitive-offset/issues
+license: BSD-3-Clause
+license-file: LICENSE
+author: Andrew Martin
+maintainer: andrew.thaddeus@gmail.com
+copyright: 2019 Andrew Martin
+category: Data
+extra-source-files: CHANGELOG.md
+
+library
+  exposed-modules:
+    Data.Primitive.PrimArray.Offset
+    Data.Primitive.ByteArray.Offset
+  build-depends:
+    , base >=4.11.1.0 && <5
+    , primitive >= 0.6.4
+  hs-source-dirs: src
+  default-language: Haskell2010
diff --git a/src/Data/Primitive/ByteArray/Offset.hs b/src/Data/Primitive/ByteArray/Offset.hs
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--- /dev/null
+++ b/src/Data/Primitive/ByteArray/Offset.hs
@@ -0,0 +1,23 @@
+{-# language DuplicateRecordFields #-}
+module Data.Primitive.ByteArray.Offset
+  ( -- * Types
+    ByteArrayOffset(..)
+  , MutableByteArrayOffset(..)
+  ) where
+
+import Control.Monad.Primitive (PrimMonad,PrimState)
+import Data.Primitive (ByteArray,MutableByteArray)
+
+-- | A byte array and an index into the array. The element
+-- type is understood to be byte (an 8-bit word).
+data ByteArrayOffset = ByteArrayOffset
+  { array :: {-# UNPACK #-} !ByteArray
+  , index :: {-# UNPACK #-} !Int
+  }
+
+-- | A mutable byte array and an index into the array. The element
+-- type is understood to be byte (an 8-bit word).
+data MutableByteArrayOffset s = MutableByteArrayOffset
+  { array :: {-# UNPACK #-} !(MutableByteArray s)
+  , index :: {-# UNPACK #-} !Int
+  }
diff --git a/src/Data/Primitive/PrimArray/Offset.hs b/src/Data/Primitive/PrimArray/Offset.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Primitive/PrimArray/Offset.hs
@@ -0,0 +1,65 @@
+{-# language DuplicateRecordFields #-}
+
+-- | Data types for describing an array paired with an index
+-- into it. This is intended to be used in wrappers for @unsafe@
+-- FFI calls. For example, the POSIX function @recvfrom@ takes
+-- a @socklen_t*@ argument. (Let us assume that @socklen_t@
+-- is equivalant to @int@ for this example.) How is this argument best
+-- described by a Haskell type? When working with pinned memory,
+-- the best option @'Ptr' 'CInt'@.
+-- It works equally well regardless of whether we originally had an
+-- array of 'CInt' or a pointer to a single 'CInt'. This works because
+-- of functions like 'Data.Primitive.advancePtr' and
+-- 'Foreign.Ptr.plusPtr' that effectively index into an array.
+-- Unpinned memory, however, is trickier. We want to have the full
+-- flexibility (handling both a single-element or
+-- multi-element buffer) that @'Ptr' 'CInt'@ affords. We cannot
+-- offset into a 'MutablePrimArray' to get a new one like we
+-- could with 'Ptr'. (Such a function is not possible because
+-- unpinned memory can be relocated.) So, the offseting must
+-- be done in the C function wrapped by the @unsafe@ FFI. This
+-- means that the offset must be passed together with the
+-- 'MutablePrimArray'. This is the precisely the product that
+-- 'MutablePrimArrayOffset' represents. In a type signature, it
+-- provides additional clarity about the meaning of the offset.
+--
+-- This library is used in the extensively in the @posix-api@
+-- library to clarify intent in a number of type signatures.
+module Data.Primitive.PrimArray.Offset
+  ( -- * Types
+    PrimArrayOffset(..)
+  , MutablePrimArrayOffset(..)
+    -- * Resolution
+  , indexOffset
+  , readOffset
+  ) where
+
+import Control.Monad.Primitive (PrimMonad,PrimState)
+import Data.Primitive (Prim,PrimArray,MutablePrimArray)
+import Data.Primitive (indexPrimArray,readPrimArray)
+
+-- | A primitive array and an index into the array.
+data PrimArrayOffset a = PrimArrayOffset
+  { array :: {-# UNPACK #-} !(PrimArray a)
+  , index :: {-# UNPACK #-} !Int
+  }
+
+-- | A mutable primitive array and an index into the array.
+data MutablePrimArrayOffset s a = MutablePrimArrayOffset
+  { array :: {-# UNPACK #-} !(MutablePrimArray s a)
+  , index :: {-# UNPACK #-} !Int
+  }
+
+-- | Recover the element in the primitive array.
+indexOffset :: Prim a
+  => PrimArrayOffset a
+  -> a
+{-# inline indexOffset #-}
+indexOffset (PrimArrayOffset arr ix) = indexPrimArray arr ix
+
+-- | Recover the element in the mutable primitive array.
+readOffset :: (PrimMonad m, Prim a)
+  => MutablePrimArrayOffset (PrimState m) a
+  -> m a
+{-# inline readOffset #-}
+readOffset (MutablePrimArrayOffset arr ix) = readPrimArray arr ix