visibility of participants in open water bright coloured swimming caps are almost exclusively used to allow swimmers to be seen for both safety and spectators. Swim goggles for open water swimming will often have either coloured or mirrored lenses, because without this light reflecting of the waters surface will dazzle or blind swimmers not allowing them to see where they are swimming. Wetsuits Because most triathlon swim stages that are conducted in open waters are cold, many entrants wear wetsuits. However, typical wetsuits manufactured for snorkeling or surfing are not optimised for swimming, because the tightness at the shoulders generally restrict the range of motion too much for comfortable stroking during the swim. Modern triathlon wetsuits were invented by Dan Empfield in 1987 and are customised to the needs of triathletes, and generally incorporate the following features: Thinner and/or more pliable rubber near the shoulders, sleeveless versions are also available. Long zippers and sometimes wrist/ankle zippers to facilitate quick removal during transition. In addition, triathlon wetsuits have a very smooth, but often fragile, surface. This slick surface helps to reduce water friction and allow a faster swim, but it would be easily destroyed by contact with a sandy surfboard. Another advantage of a wetsuit is the added buoyancy around the hips and legs which can provide competitors with a considerable advantage. Specific rules vary by event, but typically the legality of racing in a wetsuit depends on the water temperature on the day of the event. Wetsuits are disallowed if the water temperature is above a specified threshold and become mandatory if the water temperature is below and optional between the two, for example the British Triathlon rules state that wetsuits are forbidden above 22 °C and mandatory below 14 °C. If not racing in a wetsuit an athlete has the option of wearing any other style of swimsuit allowed by the rules. Under the current ITU rule set , there exists a wetsuit thickness rule. No wetsuit with a thickness of greater than 5mm may be used. Most triathlon wetsuits have thicknesses of 3mm to 5mm. Disallowed aids Any artificial propulsion device, such as fins, paddles or swim socks as well as any flotation device except a wetsuit, is prohibited. Headphones or any in ear device such as a tempo timer. Snorkels used to be a grey area with many national federations including that of Canada, Australia and the