Index: Lib/ntpath.py
===================================================================
--- Lib/ntpath.py	(revision 69123)
+++ Lib/ntpath.py	(working copy)
@@ -468,22 +468,38 @@
 supports_unicode_filenames = (hasattr(sys, "getwindowsversion") and
                               sys.getwindowsversion()[3] >= 2)
 
+def _do_split(path):
+    if path.endswith(sep):
+        # Should return [''] for '\\' instead of ['', '']
+        return path.split(sep)[:-1]
+    else:
+        return path.split(sep)
+
+def _abspath_split(path):
+    unc, rest = splitunc(abspath(path))
+    if unc:
+        return True, unc, _do_split(rest)
+    drive, tail = splitdrive(rest)
+    return False, drive, _do_split(tail)
+
 def relpath(path, start=curdir):
     """Return a relative version of a path"""
 
     if not path:
         raise ValueError("no path specified")
-    start_list = abspath(start).split(sep)
-    path_list = abspath(path).split(sep)
-    if start_list[0].lower() != path_list[0].lower():
-        unc_path, rest = splitunc(path)
-        unc_start, rest = splitunc(start)
-        if bool(unc_path) ^ bool(unc_start):
-            raise ValueError("Cannot mix UNC and non-UNC paths (%s and %s)"
-                                                                % (path, start))
+    start_is_unc, start_prefix, start_list = _abspath_split(start)
+    path_is_unc, path_prefix, path_list = _abspath_split(path)
+
+    if path_is_unc ^ start_is_unc:
+        raise ValueError("Cannot mix UNC and non-UNC paths (%s and %s)"
+                                                            % (path, start))
+    if path_prefix.lower() != start_prefix.lower():
+        if path_is_unc:
+            raise ValueError("path is on UNC name %s, start on UNC name %s"
+                                                    % (path_prefix, start_prefix))
         else:
             raise ValueError("path is on drive %s, start on drive %s"
-                                                % (path_list[0], start_list[0]))
+                                                    % (path_prefix, start_prefix))
     # Work out how much of the filepath is shared by start and path.
     for i in range(min(len(start_list), len(path_list))):
         if start_list[i].lower() != path_list[i].lower():
Index: Lib/posixpath.py
===================================================================
--- Lib/posixpath.py	(revision 69123)
+++ Lib/posixpath.py	(working copy)
@@ -386,14 +386,22 @@
 
 supports_unicode_filenames = False
 
+def _abspath_split(path):
+    path = abspath(path)
+    if path.endswith(sep):
+        # Should return [''] for '/' instead of ['', '']
+        return path.split(sep)[:-1]
+    else:
+        return path.split(sep)
+
 def relpath(path, start=curdir):
     """Return a relative version of a path"""
 
     if not path:
         raise ValueError("no path specified")
 
-    start_list = abspath(start).split(sep)
-    path_list = abspath(path).split(sep)
+    start_list = _abspath_split(start)
+    path_list = _abspath_split(path)
 
     # Work out how much of the filepath is shared by start and path.
     i = len(commonprefix([start_list, path_list]))
Index: Lib/test/test_ntpath.py
===================================================================
--- Lib/test/test_ntpath.py	(revision 69123)
+++ Lib/test/test_ntpath.py	(working copy)
@@ -177,8 +177,11 @@
         tester('ntpath.relpath("a", "b/c")', '..\\..\\a')
         tester('ntpath.relpath("//conky/mountpoint/a", "//conky/mountpoint/b/c")', '..\\..\\a')
         tester('ntpath.relpath("a", "a")', '.')
+        tester('ntpath.relpath("d:/a", "d:/")', 'a')
+        tester('ntpath.relpath("d:/a", "d:/b")', '..\\a')
+        tester('ntpath.relpath("d:/a", "d:/a/b")', '..')
+        tester('ntpath.relpath("d:/a/b", "d:/a")', 'b')
 
-
 def test_main():
     test_support.run_unittest(TestNtpath)
 
Index: Lib/test/test_posixpath.py
===================================================================
--- Lib/test/test_posixpath.py	(revision 69123)
+++ Lib/test/test_posixpath.py	(working copy)
@@ -502,6 +502,10 @@
             self.assertEqual(posixpath.relpath("a/b", "../c"), "../"+curdir+"/a/b")
             self.assertEqual(posixpath.relpath("a", "b/c"), "../../a")
             self.assertEqual(posixpath.relpath("a", "a"), ".")
+            self.assertEqual(posixpath.relpath("/a", "/"), "a")
+            self.assertEqual(posixpath.relpath("/a", "/b"), "../a")
+            self.assertEqual(posixpath.relpath("/a", "/a/b"), "..")
+            self.assertEqual(posixpath.relpath("/a/b", "/a"), "b")
         finally:
             os.getcwd = real_getcwd