diff --git a/Headers/Foundation/NSGeometry.h b/Headers/Foundation/NSGeometry.h
index dbf9f10c5..c2ff44dff 100644
--- a/Headers/Foundation/NSGeometry.h
+++ b/Headers/Foundation/NSGeometry.h
@@ -314,7 +314,10 @@ NSInsetRect(NSRect aRect, float dX, float dY) GS_GEOM_ATTR;
 
 /** Returns the rectangle obtained by moving each of aRect's
  * horizontal sides inward by dy and each of aRect's vertical
- * sides inward by dx. */
+ * sides inward by dx.<br />
+ * NB. For MacOS-X compatability, this is permitted to return
+ * a rectanglew with nagative width or height, strange as that seems.
+ */
 GS_GEOM_SCOPE NSRect
 NSInsetRect(NSRect aRect, float dX, float dY)
 {