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. This is a versatile cassette -- it's fully compatible with a Dura Ace 7800 10-speed drivetrain. It's also compatible with any Shimano 9-speed generation hub. Available in 11-23, 12-23, 12-25, and 12-27. MSRP $99.99.