The GX 11 X-Horizon Rear derailleur from SRAM. The rear derailleur uses X-Horizon technology that reduces shifting force & chain slap. The X-Actuation technology eliminates poor shifting. The roller bearing clutch for maximum stability. The 12 tooth X-Sync jockey wheels use tall square teeth edges engage the chain earlier than traditional triangle shaped teeth. The sharp & narrow tooth profile, as well as rounded chamfered edges, help manage a deflected chain.