diff --git a/Lib/dospath.py b/Lib/dospath.py
index 5f60705..652b35f 100644
--- a/Lib/dospath.py
+++ b/Lib/dospath.py
@@ -183,13 +183,19 @@
 
 
 def walk(top, func, arg):
-    """Directory tree walk.
-    For each directory under top (including top itself, but excluding
-    '.' and '..'), func(arg, dirname, filenames) is called, where
-    dirname is the name of the directory and filenames is the list
-    files files (and subdirectories etc.) in the directory.
-    The func may modify the filenames list, to implement a filter,
-    or to impose a different order of visiting."""
+    """Directory tree walk with callback function.
+
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
 
     try:
         names = os.listdir(top)
diff --git a/Lib/macpath.py b/Lib/macpath.py
index 1ef35ef..f8ca051 100644
--- a/Lib/macpath.py
+++ b/Lib/macpath.py
@@ -201,13 +201,19 @@
 
 
 def walk(top, func, arg):
-    """Directory tree walk.
-    For each directory under top (including top itself),
-    func(arg, dirname, filenames) is called, where
-    dirname is the name of the directory and filenames is the list
-    of files (and subdirectories etc.) in the directory.
-    The func may modify the filenames list, to implement a filter,
-    or to impose a different order of visiting."""
+    """Directory tree walk with callback function.
+
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
 
     try:
         names = os.listdir(top)
diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index a1baf83..ed8a2dd 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -291,11 +291,20 @@
 # or to impose a different order of visiting.
 
 def walk(top, func, arg):
-    """Directory tree walk whth callback function.
+    """Directory tree walk with callback function.
 
-    walk(top, func, arg) calls func(arg, d, files) for each directory d
-    in the tree rooted at top (including top itself); files is a list
-    of all the files and subdirs in directory d."""
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
+
     try:
         names = os.listdir(top)
     except os.error:
diff --git a/Lib/plat-riscos/riscospath.py b/Lib/plat-riscos/riscospath.py
index 8eda834..c25572b 100644
--- a/Lib/plat-riscos/riscospath.py
+++ b/Lib/plat-riscos/riscospath.py
@@ -350,11 +350,20 @@
 # Independent of host system. Why am I in os.path?
 
 def walk(top, func, arg):
-    """
-  walk(top,func,args) calls func(arg, d, files) for each directory "d" in the tree
-  rooted at "top" (including "top" itself). "files" is a list of all the files and
-  subdirs in directory "d".
-  """
+    """Directory tree walk with callback function.
+
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
+
     try:
         names= os.listdir(top)
     except os.error:
diff --git a/Lib/posixpath.py b/Lib/posixpath.py
index c587b68..c342bbc 100644
--- a/Lib/posixpath.py
+++ b/Lib/posixpath.py
@@ -258,10 +258,20 @@
 # or to impose a different order of visiting.
 
 def walk(top, func, arg):
-    """walk(top,func,arg) calls func(arg, d, files) for each directory "d"
-    in the tree  rooted at "top" (including "top" itself).  "files" is a list
-    of all the files and subdirs in directory "d".
-    """
+    """Directory tree walk with callback function.
+
+    For each directory in the directory tree rooted at top (including top
+    itself, but excluding '.' and '..'), call func(arg, dirname, fnames).
+    dirname is the name of the directory, and fnames a list of the names of
+    the files and subdirectories in dirname (excluding '.' and '..').  func
+    may modify the fnames list in-place (e.g. via del or slice assignment),
+    and walk will only recurse into the subdirectories whose names remain in
+    fnames; this can be used to implement a filter, or to impose a specific
+    order of visiting.  No semantics are defined for, or required of, arg,
+    beyond that arg is always passed to func.  It can be used, e.g., to pass
+    a filename pattern, or a mutable object designed to accumulate
+    statistics.  Passing None for arg is common."""
+
     try:
         names = os.listdir(top)
     except os.error: