The Box One Prime 9 Rear Derailleur takes the advantages of our legacy 9-speed drive-train but adds the advancements of a modern 1 by; a fully adjustable Tri-Pack™ 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. IMPORTANT NOTES Prime 9 Technology™ View Prime 9 Drivetrain Compatibility Manual How is the Ebike variant different? View Prime 9 Comparison Chart View Prime 9 E-Bike Comparison Chart Why Prime 9?