diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index 8cfec1e32..a2bbfbd1f 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -36,6 +36,7 @@
 #define HB_DEBUG_CORETEXT (HB_DEBUG+0)
 #endif
 
+/* https://developer.apple.com/documentation/coretext/1508745-ctfontcreatewithgraphicsfont */
 #define HB_CORETEXT_FONT_SIZE 12.0
 
 static void