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Caps are worn for various reasons. Some facilities require the wearing of swim caps, in order to protect filters from becoming clogged with loose hairs which fall from the head of swimmers who are not wearing a cap. Caps are also sometimes worn in an attempt to keep the hair relatively dry or free from chlorinated water, to keep the sun off the hair, and also, when a cap is worn with ear plugs, in order to keep water out of the ears.
Competitive swim caps are made of tightly fitted silicone, latex, or lycra that hugs the skull of its user, providing cover for his or her hair. This reduces drag in the water caused by loose hair. During longer swimming sessions, a swim cap keeps the wearer's head warm.
Swim caps were made of rubberized fabric during the early 20th century. By the 1920s they were made of latex. The earliest chin strap caps were known as "aviator's style" as they resembled the strapped leather helmets of flyers of the day. During the 1940s swim caps became scarce as rubber was needed for war materials. It was a lucky girl who had a swim cap to protect her "wave" (hair style) during that period. The permanent wave hairstyle took time to obtain and was expensive, so many women wanted to protect their hair while swimming. The 1950s saw decorated caps come into vogue, and during the 1960s colorful flower petal swim caps became popular. Men's long hair styles of the late 1960s and early 1970s made swimming pool operators change rules requiring swim caps for swimmers with long hair. Without swim cap requirements wearing swim caps fell out of fashion during the early 1970s. Competitive swimming in the 1980s and 1990s and the worldwide construction of indoor lap swimming pools for fitness swimming, made the swim cap popular again. Today swim caps are available in competitive racing styles, traditional chin strap type with inner sealing to keep out water and colorful floral styles reminiscent of the 1960s as well as other graphic prints.
Double swim caps 
Many swimmers have trouble finding a swim cap that keeps their hair dry. "Double capping"—wearing two swim caps—can provide a tighter swim cap seal. An inner silicon or latex swim cap pulled low over the ears worn under a second traditional style chin strap swim cap with an inner seal may provide the protection desired. "Double capping" is also used by participants of open water swimming to provide warmth.
During the 2012 Olympics held in London U.K., some swimmers wore double swim caps. Swimmers make this choice because the inner cap is latex and grips the head, their swim goggles go over the latex swim cap, and a second snug silicon swim cap is fitted over the goggles and inner cap making as smooth a surface as possible for maximum streamlining.
English Channel swimming rules for swim caps 
The English "Channel Swimming and Piloting Federation" and the "Channel Swimming Association" state that "one standard swim cap" may be used for official English Channel record swims. The cap must be approved by the official observer who accompanies the swimmer on the pilot boat across the channel. "Standard swim caps" that have been used by English Channel swimmers of record over the years include solid rubber "tank" style swim caps, molded rubber swim caps with chin strap, bubble crepe rubber swim caps with chin strap, silicone swim caps and latex "racing" swim caps. Swim caps that have neoprene within the cap construction such as the "Barracuda Hothead", or that are completely made of neoprene are not allowed for record English Channel swims. Several other long distance swims such as the Santa Catalina Channel California swim have similar rules for record swims. Swim caps worn for open water swims should be of visible colors that contrast with the water so that observers may safely monitor them, and operators of vessels may see them.
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