Triathletes will often use their legs less vigorously and more carefully than other swimmers, conserving their leg muscles for the cycle and run to follow. Many triathletes use altered swim strokes to compensate for turbulent, aerated water and to conserve energy for a long swim. In addition, the majority of triathlons involve open-water (outdoor) swim stages, rather than pools with lane markers. As a result, triathletes in the swim stage must jockey for position, and can gain some advantage by drafting, following a competitor closely to swim in their slipstream. Triathletes will often use "dolphin kicking" and diving to make headway against waves, and body surfing to use a wave's energy for a bit of speed at the end of the swim stage. Also, open-water swims necessitate "sighting": raising the head to look for landmarks or buoys that mark the course. A modified stroke allows the triathlete to lift the head above water to sight without interrupting the swim or wasting energy. Because open water swim areas are often cold and because wearing a wetsuit provides a competitive advantage, specialized triathlon wetsuits have been developed in a variety of styles to match the conditions of the water. For example, wetsuits that are sleeveless and cut above the knee are designed for warmer waters, while still providing buoyancy. Wetsuits are legal in sanctioned events at which the surface water temperature is 78 °F (26 °C) or less. In non-sanctioned events or in "age group" classes where most racers are simply participating for the enjoyment of the sport instead of vying for official triathlon placing, wetsuits can often be used at other temperatures. Race directors will sometimes discourage or ban wetsuits if the water temperature is above 84 °F (29 °C) due to overheating that can occur while wearing a wetsuit. Other rules have been implemented by race organizers regarding both wetsuit thickness as well as the use of "swim skins;" which need to be considered by those participating in future triathlons. Some triathlon sanctioning bodies have placed limits on the thickness of the wetsuit material. Under ITU and some national governing bodies' rules no wetsuit may have a thickness of more than 5 mm (0.20 in).