Document key-functions in the glossary.
diff --git a/Doc/glossary.rst b/Doc/glossary.rst
index 7070918..24bbb37 100644
--- a/Doc/glossary.rst
+++ b/Doc/glossary.rst
@@ -362,6 +362,26 @@
value is assigned. ``**`` is used to accept or pass a dictionary of
keyword arguments. See :term:`argument`.
+ key function
+ A key function or collation function is a callable that returns a value
+ used for sorting or ordering. For example, :func:`locale.strxfrm` is
+ used to produce a sort key that is aware of locale specific sort
+ conventions.
+
+ A number of tools in Python accept key functions to control how elements
+ are ordered or grouped. They include :func:`min`, :func:`max`,
+ :func:`sorted`, :meth:`list.sort`, :func:`heapq.nsmallest`,
+ :func:`heapq.nlargest`, and :func:`itertools.groupby`.
+
+ There are several ways to create a key function. For example. the
+ :meth:`str.lower` method can serve as a key function for case insensitive
+ sorts. Alternatively, an ad-hoc key function can be built from a
+ :keyword:`lambda` expression such as ``lambda r: (r[0], r[2])``. Also,
+ the :mod:`operator` module provides three key function constuctors:
+ :func:`~operator.attrgetter`, :func:`~operator.itemgetter`, and
+ :func:`~operator.methodcaller`. See the :ref:`sorting-howto` for
+ examples of how to create and use key functions.
+
lambda
An anonymous inline function consisting of a single :term:`expression`
which is evaluated when the function is called. The syntax to create
diff --git a/Doc/howto/sorting.rst b/Doc/howto/sorting.rst
index 79008a7..5acd405 100644
--- a/Doc/howto/sorting.rst
+++ b/Doc/howto/sorting.rst
@@ -1,3 +1,5 @@
+.. _sorting-howto:
+
Sorting HOW TO
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