skates clockwise on the track toward the rear of the engagement zone to maximize the time the jammer must spend before returning to action. Bridging: By separating up to 10 feet, blockers can stretch both the pack and the engagement zone, allowing teammates to keep hindering the opposition jammer. For example, in the strategy of running back (see above), coordinated action by the four skaters other than the jammer could force the opponent jammer to detour a full 40 feet before returning to action. Officials WFTDA bouts are officiated by three to seven skating referees and many non-skating officials (NSOs). Volunteer leagues adapt when fewer than the optimal number of officials are present. Referees Up to four referees skate on the inside of the track. In flat-track derby, up to three additional referees skate on the outside of the track. They call penalties, award points, and ensure safe game play. Referees must wear skates and typically wear white and black stripes. Non-skating officials (NSOs) NSOs take up a range of positions inside and outside the track, start and time the jams, record and display scores and penalties communicated by referees, record the number of each skater on track for a given jam, and time and record skaters in the penalty box. History Professional endurance races The growing popularity of roller skating in the United States led to the formation of organized multi-day endurance races for cash prizes as early as the mid-1880s. Speed and endurance races continued to be held on both flat and banked tracks in the century's first three decades and spectators enjoyed the spills and falls of the skaters. The term derby was used to refer to such races by 1922. Evolution to contact sport The endurance races began to transform into the contemporary form of the sport in the mid-1930s, when promoter Leo Seltzer created the Transcontinental Roller Derby, a month-long simulation of a road race between two-person teams of professional skaters. The spectacle became a popular touring exhibition. In the late 1930s, sportswriter Damon Runyon persuaded Seltzer to change the Roller Derby rules to increase skater contact. By 1939, after experimenting with different team and scoring arrangements, Seltzer's created a touring company of four pairs of teams (always billed as the local "home" team versus either New York or Chicago), with two five-person teams on the track at once, scoring points when its members lapped