Portamate PM-3036 Extension Rails for PM-3500 Mobile base (2 Piece), 36" Includes two 36" extension rails and assembly hardware Easy assembly Substitute 20" rail with 36" rail for a max size of 18-1/4" x 34-1/4" Substitute 24" rail with 36" rail for a max size of 33-1/2" x 44"