Abstract: A seating system for use with a base such as a conventional, preexisting wheelchair base includes a support unit and mounting bracket assemblies for removably mounting the support unit to the base. The support unit includes a frame assembly and a plurality of cushion elements adjustably connected to the frame assembly. The frame assembly includes a horizontal frame selectively adjustable in width to accommodate wheelchairs of varying widths. The frame assembly also includes a vertical frame, or spine, and an attachment mechanism for removably attaching the spine to the horizontal frame. The spine is adjustable both fore and aft and transversely. The cushion elements each include a connection device for independently and adjustably connecting the cushion elements to the frame assembly. The cushion elements are contourable so that they may be formed and reformed to fit to the specialized needs of each individual using the system.