Thule’s 503 Load Stops are vertical rack attachments designed to stabilize and prevent side to side load shifting. Thule 503 Load Stops are ideal for stabilizing light loads of lumber, ladders, pipe, canoes, etc. Thule 503 Load Stops are designed to fit standard Thule load Bars and will not fit Thule AeroBlade load Bars. The 503 Load Stop set includes 4 load stops and all associated fastening hardware. Thule Heavy-duty Cam Buckle Tie down Straps are recommended for securing your load.