Once made of highly polished wood, most skis are now made of plastics, polyurethane foam, and other materials. They come in many different sizes and styles, depending on whether their intended use is cross-country, downhill, ski jumping, freeskiing, or another branch of the sport. Traditional skis have an upturned tip at one end only; twin-tip skis, with upturned tips at both ends, were originally used in freeskiing, but versions are available for other uses. The bindings that attach the ski to the boot vary as well. Most skiers also use a pair of poles, each of which has a wriststrap on the top, a sharp tip on the bottom, and a circular ring about 4 in. (10 cm) from the tip to prevent it from sinking into the snow. The bottoms of skis may be waxed for maximum glide in varying snow conditions.