diff --git a/src/sfnt/sfdriver.c b/src/sfnt/sfdriver.c
index 460fc9efa..9f7a327bf 100644
--- a/src/sfnt/sfdriver.c
+++ b/src/sfnt/sfdriver.c
@@ -464,7 +464,12 @@
   typedef int (*char_type_func)( int  c );
 
 
-  /* handling of PID/EID 3/0 and 3/1 is the same */
+  /*
+   * Handling of PID/EID 3/0 and 3/1 is the same.  The language ID
+   * restriction to use value 0x409 (US English) comes from OpenType's
+   * recommendation section
+   * (https://docs.microsoft.com/en-us/typography/opentype/spec/recom#name).
+   */
 #define IS_WIN( n )  ( (n)->platformID == 3                             && \
                        ( (n)->encodingID == 1 || (n)->encodingID == 0 ) && \
                        (n)->languageID == 0x409                         )