Actually it's the thread diameter and 1 tad = 0.734mm or 0.0289". Since the thread pitch is 24 tpi on both of the sizes, it's easy to mistakingly use a Miche or Campy or Phil lockring on a hub meant for the smaller (1.29x24) hub standard, and strip the threads when tightening.

Here's a typical installation >>> http://www.worldclasscycles.com/mich...ier_on_hub.htm

Note the use of a Dura Ace lockring. This is how mine is set up.