diff --git a/src/hb-aat-layout-kerx-table.hh b/src/hb-aat-layout-kerx-table.hh
index d6747f9a8..e5934c39f 100644
--- a/src/hb-aat-layout-kerx-table.hh
+++ b/src/hb-aat-layout-kerx-table.hh
@@ -317,7 +317,7 @@ struct kerx
         goto skip;
 
       if (table->coverage & KerxTable::CrossStream)
-        goto skip; /* We do NOT handle cross-stream kerning. */
+        goto skip; /* We do NOT handle cross-stream kerning.  None of Apple fonts use it. */
 
       reverse = bool (table->coverage & KerxTable::Backwards) !=
 		HB_DIRECTION_IS_BACKWARD (c->buffer->props.direction);
diff --git a/src/hb-ot-kern-table.hh b/src/hb-ot-kern-table.hh
index c4e53fa6e..ccb666e8c 100644
--- a/src/hb-ot-kern-table.hh
+++ b/src/hb-ot-kern-table.hh
@@ -463,6 +463,7 @@ struct kern
 		       hb_buffer_t  *buffer,
 		       hb_mask_t kern_mask) const
     {
+      /* We only apply horizontal kerning in this table. */
       if (!HB_DIRECTION_IS_HORIZONTAL (buffer->props.direction))
         return;