diff --git a/mlir/docs/LangRef.md b/mlir/docs/LangRef.md
--- a/mlir/docs/LangRef.md
+++ b/mlir/docs/LangRef.md
@@ -443,7 +443,8 @@
 region is as follows:
 
 ```
-region ::= `{` block* `}`
+region      ::= `{` entry-block? block* `}`
+entry-block ::= operation+
 ```
 
 A function body is an example of a region: it consists of a CFG of blocks and
@@ -454,6 +455,11 @@
 function arguments must match the types and count of the region arguments. In
 general, operations with regions can define these correspondences arbitrarily.
 
+An *entry block* is a block with no label and no arguments that may occur at
+the beginning of a region. It enables a common pattern of using a region to
+open a new scope.
+
+
 ### Value Scoping
 
 Regions provide hierarchical encapsulation of programs: it is impossible to