diff --git a/new/feature.jam b/new/feature.jam
index 2c43bdf5a..f33c16dcf 100644
--- a/new/feature.jam
+++ b/new/feature.jam
@@ -312,13 +312,20 @@ local rule validate-value-string ( feature value-string )
 }
 
 # Extends the given subfeature with the subvalues.  If the optional
-# value-string is provided, the subvalues are specific to the given
+# value-string is provided, the subvalues are only valid for the given
 # value of the feature. Thus, you could say that
 # <target-platform>mingw is specifc to <toolset>gcc-2.95.2 as follows:
 #
 #       extend-subfeature toolset gcc-2.95.2 : target-platform : mingw ;
 #
-rule extend-subfeature ( feature value-string ? : subfeature : subvalues * )
+rule extend-subfeature ( 
+  feature           # The feature whose subfeature is being extended
+    
+    value-string ?  # If supplied, specifies a specific value of the
+                    # main feature for which the subfeature values are valid
+    : subfeature    # The name of the subfeature
+    : subvalues *   # The additional values of the subfeature being defined.
+)
 {
     feature = [ grist $(feature) ] ;
     validate-feature $(feature) ;
diff --git a/v2/build/feature.jam b/v2/build/feature.jam
index 2c43bdf5a..f33c16dcf 100644
--- a/v2/build/feature.jam
+++ b/v2/build/feature.jam
@@ -312,13 +312,20 @@ local rule validate-value-string ( feature value-string )
 }
 
 # Extends the given subfeature with the subvalues.  If the optional
-# value-string is provided, the subvalues are specific to the given
+# value-string is provided, the subvalues are only valid for the given
 # value of the feature. Thus, you could say that
 # <target-platform>mingw is specifc to <toolset>gcc-2.95.2 as follows:
 #
 #       extend-subfeature toolset gcc-2.95.2 : target-platform : mingw ;
 #
-rule extend-subfeature ( feature value-string ? : subfeature : subvalues * )
+rule extend-subfeature ( 
+  feature           # The feature whose subfeature is being extended
+    
+    value-string ?  # If supplied, specifies a specific value of the
+                    # main feature for which the subfeature values are valid
+    : subfeature    # The name of the subfeature
+    : subvalues *   # The additional values of the subfeature being defined.
+)
 {
     feature = [ grist $(feature) ] ;
     validate-feature $(feature) ;