This is Shimano's XTR M9100 Rear Derailleur. The extra cog of 12 speed gives you more range and allows for smaller jumps between cogs to help you find the right gear. Features: Available in two cage sizes. The medium cage provides 28mm of extra ground clearance Decreased tension at low gear position to prevent high chain tension Bigger 13T pulleys Shadow Plus clutch reduces chain slap Specifications: Speeds: 1 x 12 Max Cog: 45T (Medium Cage), 51T (Long Cage) Min Cog: 10T Weight: 237g (Medium Cage), 240g (Long Cage)