diff --git a/packaging/markers.py b/packaging/markers.py
index 3b8af324..708dd9a2 100644
--- a/packaging/markers.py
+++ b/packaging/markers.py
@@ -85,13 +85,13 @@ def serialize(self):
     | L("python_version")
     | L("sys_platform")
     | L("os_name")
-    | L("os.name")
+    | L("os.name")  # PEP-345
     | L("sys.platform")  # PEP-345
     | L("platform.version")  # PEP-345
     | L("platform.machine")  # PEP-345
     | L("platform.python_implementation")  # PEP-345
-    | L("python_implementation")  # PEP-345
-    | L("extra")  # undocumented setuptools legacy
+    | L("python_implementation")  # undocumented setuptools legacy
+    | L("extra")  # PEP-508
 )
 ALIASES = {
     "os.name": "os_name",