diff --git a/docs/text-sizing-protocol.rst b/docs/text-sizing-protocol.rst
index 95690e219..8a63e5ca9 100644
--- a/docs/text-sizing-protocol.rst
+++ b/docs/text-sizing-protocol.rst
@@ -461,8 +461,9 @@ Unicode variation selectors
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 There are two code points (:code:`U+FE0E` and :code:`U+FE0F`) that can actually
-alter the width of the previous code point. When adding a code point to the
-previous cell these have to be handled specially.
+`alter the width of the previous code point <http://www.unicode.org/reports/tr11/#ED4>`__
+by changing their presentation between *Emoji_Presentation* and *Text_Presentation*.
+When adding a code point to the previous cell these have to be handled specially.
 
 ``U+FE0E`` - Variation Selector 15
   When the previous cell has width two and the last code point in the previous