diff -r f2e6c33ce3e9 Doc/tutorial/datastructures.rst
--- a/Doc/tutorial/datastructures.rst	Thu Jul 03 10:57:44 2014 -0500
+++ b/Doc/tutorial/datastructures.rst	Fri Jul 04 14:34:42 2014 +0200
@@ -74,6 +74,12 @@
    Sort the items of the list in place (the arguments can be used for sort
    customization, see :func:`sorted` for their explanation).
 
+   Each item needs to have an ordering relationship with the other items;
+   otherwise, :exc:`TypeError` is raised. An ordering relationship for
+   user-defined classes can be established by defining
+   :dfn:`rich comparison methods` like :meth:`__lt__` or the comparison method
+   :meth:`__cmp__`, both described in section :ref:`customization`.  For details
+   on the ordering relationships of built-in types, see :ref:`comparisons`.
 
 .. method:: list.reverse()
    :noindex: