The bottom 4 cogs on the Dura-Ace 10-Speed cassette are made from titanium -- unlike the 2 (or sometimes 3, depending on ratio) you got in the 9-speed era. These Ti cogs get an improved surface treatment to give them a 30% increase in wear. They are compatible with all 8 and 9-speed Shimano hubs. Comes with a lockring. Choose 11/21 or 11/23. MSRP $199.99.