diff --git a/relational-record.cabal b/relational-record.cabal
--- a/relational-record.cabal
+++ b/relational-record.cabal
@@ -1,5 +1,5 @@
 name:                relational-record
-version:             0.1.4.0
+version:             0.1.5.0
 synopsis:            Meta package of Relational Record
 description:         Meta package to install Relational Record quickly
 homepage:            http://khibino.github.io/haskell-relational-record/
diff --git a/src/Database/Relational/Query/Documentation.hs b/src/Database/Relational/Query/Documentation.hs
--- a/src/Database/Relational/Query/Documentation.hs
+++ b/src/Database/Relational/Query/Documentation.hs
@@ -111,6 +111,8 @@
 
   -- ** Placeholders
   -- $placeholders
+  placeholder,
+
   query',
   left',
   relation',
@@ -319,6 +321,10 @@
  -}
 
 {- $placeholders
+'placeholders' operator takes
+a lambda-form which argument is 'Projection' typed placeholders and its scope is restricted by that lambda-form
+and then creates dummy value with 'Placeholders' typed which propagate placeholder type information into 'Relation' layer.
+
 Placeholders' flavor of operators against query operation and set operation are also provided, to realize type safe placeholders.
 
 'query'', 'left'', 'relation'', 'updateTarget'', 'restriction'', and 'union''