Roller derby is a roller skating contact sport played by two teams of fifteen members. Roller derby is played by approximately 1,250 amateur leagues worldwide, mostly in the United States. Game play consists of a series of short scrimmages (jams) in which both teams designate a jammer (who uniquely wears a star on the helmet) and four blockers to skate counter-clockwise around a track. The jammer scores points by lapping members of the opposing team. The teams attempt to hinder the opposing jammer while assisting their own jammer—in effect, playing both offense and defense simultaneously. Overview While the sport has its origins in the banked-track roller-skating marathons of the 1930s, Leo Seltzer and Damon Runyon are credited with evolving the sport to its competitive form. Professional roller derby quickly became popular; in 1940, more than 5 million spectators watched in about 50 American cities. In the ensuing decades, however, it predominantly became a form of sports entertainment, where theatrical elements overshadowed athleticism. Gratuitous showmanship largely ended with the sport's grassroots revival in the first decade of the 21st century. Although roller derby retains some sports entertainment qualities such as player pseudonyms and colorful uniforms, it has abandoned scripted bouts with predetermined winners. Modern roller derby is an international sport, mostly played by amateurs. It was under consideration as a roller sport for the 2020 Summer Olympics. Fédération Internationale de Roller Sports (FIRS), recognized by the International Olympic Committee as the official international governing body of roller sports, released its first set of Roller Derby Rules for the World Roller Games, organised by World Skate, that took place September 2017 in Nanjing, China. Most modern leagues (and their back-office volunteers) share a strong "do-it-yourself" ethic that combines athleticism with the styles of punk and camp. , the Women's Flat Track Derby Association (WFTDA) had 451 full member leagues and 46 apprentice leagues and the Roller Derby Coalition of Leagues (RDCL) supporting women's banked track roller derby had eight full member leagues. Rules Contemporary roller derby has a basic set of rules, with variations reflecting the interests of a governing body's member leagues. The summary below is based on the rules of the Women's Flat Track Derby Association (WFTDA). In March 2010, Derby News Network said that more than 98% of roller derby competitions were conducted under WFTDA rules. For example, members of the United Kingdom Roller Derby Association are required to