Ski geometry is the shape of the ski. Described in the direction of travel, the front of the ski, typically pointed or rounded, is the tip, the middle is the waist and the rear is the tail. Skis have four aspects that define their basic performance: length, width, sidecut and camber. Skis also differ in more minor ways to address certain niche roles. For instance, skis for moguls are much softer to absorb shocks from the quick and sharp turns of the moguls and skis for powder are much wider to provide more "float" in deeper, softer snow. Length and width The length and width of the ski define its total surface area, which provides some indication of the ski's float, or ability to remain on top of the snow instead of sinking into it. Cross-country skis must be narrow to reduce drag, and thus must be long to produce the required float. Alpine skis are generally not designed to reduce drag, and tend to be shorter and wider. Skis used in downhill race events are longer, with a subtle side cut, built for speed and wide turns. Slalom skis, as well as many recreational skis, are shorter with a greater side cut to facilitate tighter, easier turns. For off-piste skis the trend is towards wider skis that better float on top of powder snow. The ski width of all-mountain and off-piste skis has generally increased since the 1990s when 85 mm width was considered a wide powderski. From 2010 and onwards, many well known ski manufacturers sell all-round freeride skis for the general public starting in the 90mm range and going up to 120 mm or more. Tips and tails The tip of the ski often strikes the snow and is normally curled upward in order to ride over it. Tips were pointed for much of the history of skiing, but the introduction of wider shaped skis has led to a change to more rounded shapes. Tails were, and often remain, straight cut. For freestyle skiing, where the skier is often skiing backwards, it is common to have a "twin-tip" design with the tail of the ski rounded and curled up like the nose so that it skis the same in both directions. One design note that makes a periodic comeback is the "swallowtail" design, where a notch, often V-shaped, is cut out of the rear of the ski.