diff --git a/include/freetype/freetype.h b/include/freetype/freetype.h
index 3966278..ba3950e 100644
--- a/include/freetype/freetype.h
+++ b/include/freetype/freetype.h
@@ -958,6 +958,9 @@
    *     Note that the bounding box might be off by (at least) one pixel for
    *     hinted fonts.  See @FT_Size_Metrics for further discussion.
    *
+   *     Note that the bounding box does not vary in OpenType variable fonts
+   *     and should only be used in relation to the default instance.
+   *
    *   units_per_EM ::
    *     The number of font units per EM square for this face.  This is
    *     typically 2048 for TrueType fonts, and 1000 for Type~1 fonts.  Only