The BOSS heavy duty floor machine aka Floor Buffer gives you the option of interchangeable machine aprons 13, 15, 17, 19, and 21. The five interchangeable aprons with a single base machine gives you a fleet of floor machines for a range of tasks. Note: Must add 1 apron to complete machine. 13 add A-163-1, 15 add A-167-1, 17 add A-169-1, 19 add A-171-1, 20 and 21 add A-172-1. Must order properly sized pad driver separately to complete machine.