The Super Chock is much more effective than 2x4 chunks at keeping trailers and tow rigs in place (not for use on tractor-trailers or other heavy-duty commercial vehicles). Ribs on the top and bottom surfaces provide traction. Light weight (only 2 pounds) won't weigh down your trailer. Convenient molded-in handle plus a hole for hanging or tying two units together. High-visibility yellow is hard to lose or leave behind. Sold individually.