diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index ba896212925d89..58287fccc87252 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -2376,7 +2376,8 @@ expression support in the :mod:`re` module).
 
    Return a copy of the string with leading characters removed.  The *chars*
    argument is a string specifying the set of characters to be removed.  If omitted
-   or ``None``, the *chars* argument defaults to removing whitespace.  The *chars*
+   or ``None``, the *chars* argument defaults to removing whitespace, that is
+   characters for which :meth:`str.isspace` is true.  The *chars*
    argument is not a prefix; rather, all combinations of its values are stripped::
 
       >>> '   spacious   '.lstrip()
@@ -2579,7 +2580,8 @@ expression support in the :mod:`re` module).
 
    Return a copy of the string with trailing characters removed.  The *chars*
    argument is a string specifying the set of characters to be removed.  If omitted
-   or ``None``, the *chars* argument defaults to removing whitespace.  The *chars*
+   or ``None``, the *chars* argument defaults to removing whitespace, that is
+   characters for which :meth:`str.isspace` is true.  The *chars*
    argument is not a suffix; rather, all combinations of its values are stripped.
    For example: