The 312 is a bag specifically designed to fit custom bikes and choppers. It is made to attach to the swing arm with leather belts. It measures 10" x 11.5" x 3" and comes as a single bag. It also comes in the fatter version 312X which measures 10" x 11.5" x 5". It usually works on left sides unless there is a left side mounted license plate. It may work on the right side as well. Works on Harley Davidson Softails, Yamahas and Kawasaki. The 312X measures 10x11.5x5.