diff --git a/Lib/base64.py b/Lib/base64.py
index 844907f..38bc61e 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -65,8 +65,9 @@
     alternative alphabet used instead of the '+' and '/' characters.
 
     The decoded string is returned.  A TypeError is raised if s is
-    incorrectly padded.  Non-base64-alphabet characters are discarded prior
-    to the padding check.
+    incorrectly padded.  Characters that are neither in the normal base-64
+    alphabet nor the alternative alphabet are discarded prior to the padding
+    check.
     """
     if altchars is not None:
         s = s.translate(string.maketrans(altchars[:2], '+/'))
@@ -87,9 +88,10 @@
 def standard_b64decode(s):
     """Decode a string encoded with the standard Base64 alphabet.
 
-    s is the string to decode.  The decoded string is returned.  A TypeError
-    is raised if the string is incorrectly padded or if there are non-alphabet
-    characters present in the string.
+    Argument s is the string to decode.  The decoded string is returned.  A
+    TypeError is raised if the string is incorrectly padded.  Characters that
+    are not in the standard alphabet are discarded prior to the padding
+    check.
     """
     return b64decode(s)
 
@@ -97,19 +99,20 @@
 _urlsafe_decode_translation = string.maketrans(b'-_', b'+/')
 
 def urlsafe_b64encode(s):
-    """Encode a string using a url-safe Base64 alphabet.
+    """Encode a string using the URL- and filesystem-safe Base64 alphabet.
 
-    s is the string to encode.  The encoded string is returned.  The alphabet
-    uses '-' instead of '+' and '_' instead of '/'.
+    Argument s is the string to encode.  The encoded string is returned.  The
+    alphabet uses '-' instead of '+' and '_' instead of '/'.
     """
     return b64encode(s).translate(_urlsafe_encode_translation)
 
 def urlsafe_b64decode(s):
-    """Decode a string encoded with the standard Base64 alphabet.
+    """Decode a string using the URL- and filesystem-safe Base64 alphabet.
 
-    s is the string to decode.  The decoded string is returned.  A TypeError
-    is raised if the string is incorrectly padded or if there are non-alphabet
-    characters present in the string.
+    Argument s is the string to decode.  The decoded string is returned.  A
+    TypeError is raised if the string is incorrectly padded.  Characters that
+    are not in the URL-safe base-64 alphabet, and are not a plus '+' or slash
+    '/', are discarded prior to the padding check.
 
     The alphabet uses '-' instead of '+' and '_' instead of '/'.
     """
@@ -267,7 +270,7 @@
     a lowercase alphabet is acceptable as input.  For security purposes, the
     default is False.
 
-    The decoded string is returned.  A TypeError is raised if s were
+    The decoded string is returned.  A TypeError is raised if s is
     incorrectly padded or if there are non-alphabet characters present in the
     string.
     """