This set includes a pair of sprockets for a Brompton rear wheel: one (1) 12 tooth sprocket and one (1) 16 tooth sprocket, one (1) aluminum spacer, one (1) snapring, and one (1) black plastic guard to protect the hub from overshifting. These sprockets fit a 3/32" chain and require a 9 spline (i.e. Shimano style) freehub to install. They are for a 2 speed model Brompton. Instructions are included with the parts and they're available here.