Thanks to its use of thin sprockets and spacers, the Ultegra CS-6600 cassette's 10-cog stack is no thicker than the CS-6500 9-speed version. It's fully compatible with a Dura Ace 10-speed drivetrain, and it's also compatible with any Shimano 9-speed generation hub, as well as standard 130mm dropout spacing. Available in 11-23, 12-23, 12-25, and 12-27. MSRP $99.99.