diff --git a/include/freetype/ftglyph.h b/include/freetype/ftglyph.h
index 5869bc1ce..db1a2c8ba 100644
--- a/include/freetype/ftglyph.h
+++ b/include/freetype/ftglyph.h
@@ -231,6 +231,12 @@ FT_BEGIN_HEADER
   /* <Return>                                                              */
   /*    FreeType error code.  0~means success.                             */
   /*                                                                       */
+  /* <Note>                                                                */
+  /*    Because `*aglyph->advance.x' and '*aglyph->advance.y' are 16.16    */
+  /*    fixed-point numbers, `slot->advance.x' and `slot->advance.y'       */
+  /*    (which are in 26.6 fixed-point format) must be in the range        */
+  /*    ]-32768;32768[.                                                    */
+  /*                                                                       */
   FT_EXPORT( FT_Error )
   FT_Get_Glyph( FT_GlyphSlot  slot,
                 FT_Glyph     *aglyph );