If you ever need to create a Drupal core patch, you may need to check-out main branch of the appropriate version. CVS URLs for those are not easy to find, so here they are:

Drupal 5:\

cvs -d:pserver:anonymous:anonymous@cvs.drupal.org:/cvs/drupal -z6 checkout -d drupal-5 -r DRUPAL-5 drupal

Drupal 6:\

cvs -d:pserver:anonymous:anonymous@cvs.drupal.org:/cvs/drupal -z6 checkout -d drupal-6 -r DRUPAL-6 drupal

Drupal-7 (Until there's Drupal8):\

cvs -d:pserver:anonymous:anonymous@cvs.drupal.org:/cvs/drupal -z6 checkout -d drupal-7 -r MAIN drupal