the pace quickens, the pacer will usually depart the track with between one and two laps remaining, though the actual location where the pacer leaves varies with every race. With laps remaining, officials begin sounding a bell or gong, increasing in frequency until the bicyclists come around to begin the final lap. Keirin ovals are divided into specific areas: The two straightaways (homestretch and backstretch), the four turns (corners), and two locations called the "centre", referring to the area between corners 1 and 2 (1 centre) and corners 3 and 4 (2 centre). The race is monitored by referees. Two of the referees are stationed in towers along the backstretch (2nd and 3rd corners), while others review the homestretch area from a control room using closed-circuit cameras. Once the race has finished, a referee can signal a possible rule violation by illuminating a red light at the corner nearest to where the infraction may have occurred, or by waving a red flag. Judges then examine the video of the race and decide if a competitor committed a rules violation and should be disqualified. Once the order of finish is finalised, the race is declared official and the winning bets are paid. Ranks There are a total of six ranks that competitors can obtain in Japanese keirin racing. SS is the highest rank, followed by S1, S2, A1, A2 and A3. All new keirin graduates begin their careers with an A3 rank and work their way up by competing in keirin events. The color of the shorts worn by each keirin competitor indicates rank. Those in A-class (A1, A2, A3) wear black shorts with a green stripe and white stars. S-class competitors (S1 and S2) wear a red stripe instead of a green stripe. Those in the elite SS class wear red shorts with a black stripe, white stars and special insignia. Introduced in 2007, the SS ranking is assigned by the NJS every December to the top nine Keirin athletes. These nine compete in that year's Keirin Grand Prix and retain their rank until the following December. Distances The distance of each race depends on gender and rank. For men, distances for those ranked A3 are at 1,600 meters, while all others compete at 2,000 meters. The finals of some of the top graded events are run at a longer distance of 2,400 meters. The season-ending Keirin Grand Prix is