Water skiing is a sport which is played in the surface of the water. Water skiing was developed by Ralph Samuelson in the 1920's. He used a pair of boards to ski, and a clothesline as a towrope. Usually, a rider is pulled behind a boat or pulled by a cable ski installation. Riders skim the surface using one (slalom) or two skis. Most of the time, the double skis are used by beginners, and then they progress to a single ski. Waterskiing is usually played in lakes, rivers, and sometimes the ocean. There are rubber-molded bindings on water skis to keep the feet in place. Doubles skis has a single binding for each one. Waterskiing can be done by more than two persons at the same time. Water skiing usually begins in deep water, though they can also have a "dry start" which starts on the shore.