diff --git a/Help/variable/CMAKE_OSX_ARCHITECTURES.rst b/Help/variable/CMAKE_OSX_ARCHITECTURES.rst
index f660df9808..3e9e4275e9 100644
--- a/Help/variable/CMAKE_OSX_ARCHITECTURES.rst
+++ b/Help/variable/CMAKE_OSX_ARCHITECTURES.rst
@@ -7,4 +7,8 @@ This variable is used to initialize the :prop_tgt:`OSX_ARCHITECTURES`
 property on each target as it is created.  See that target property
 for additional information.
 
+If ``CMAKE_OSX_ARCHITECTURES`` is not set, the compiler's default target
+architecture is used.  For compilers provided by Xcode, this is the host
+machine's architecture.
+
 .. include:: include/CMAKE_OSX_VARIABLE.rst