The Box One Prime 9 Rear Derailleur takes the advantage of BOX’s legacy 9-speed drivetrain but adds the advancements of a modern 1x; a fully adjustable Tri-Pack(TM) Clutch, and an x-wide cage optimized for an 11-50T cassette. Forged construction steps up the durability and performance to an unparalleled level. Suited for the most demanding terrain.