A shot clock is used in basketball to quicken the pace of the game. The shot clock times a play and provides that a team on offense that does not promptly try to score points loses possession of the ball. It is distinct from the game clock, which times the entire game. The shot clock may be referred to by its initial value. For example, in the National Basketball Association (NBA), it may be called the "24-second clock".
A shot clock is also used in snooker, men's lacrosse, water polo, korfball, and ten-pin bowling. It is analogous with the play clock used in American and Canadian football, and the pitch clock used in baseball.
The shot clock is a digital clock that displays a number of seconds. The shot clock is usually displayed above the backboard behind each goal, allowing offensive players to see precisely how much time they have to shoot and officials to easily determine whether buzzer beaters should be counted. The NBA specifies that a transparent shot clock and game clock be part of the backboard assembly, and FIBA, Euroleague, and many venues use this arrangement.
Three signals indicate when the time to shoot has expired:
- A value of 0.0 on the shot clock itself
- An audible horn distinct from the scoreboard operator's signal for end of period and substitutions
- A yellow strip of lights ( LEDs) on the backboard. Both the NBA (since 2011) and FIBA (since July 2018) require this.
The NBA has had a 24-second limit since 1954. FIBA introduced a 30-second shot clock in 1956 and switched to 24 seconds in 2000. The Women's National Basketball Association (WNBA) had a 30-second clock originally and switched to 24 seconds in 2006. College basketball for both men and women has a 30-second limit.
The NBA had problems attracting fans (and positive media coverage) before the shot clock's inception. :23–31 Teams in the lead were running out the clock, passing the ball incessantly. The trailing team could do nothing but commit fouls to recover possession following the free throw. Frequent low-scoring games with many fouls bored fans. The most extreme case occurred on November 22, 1950, when the Fort Wayne Pistons defeated the Minneapolis Lakers by a record-low score of 19–18, including 3–1 in the fourth quarter.  The Pistons held the ball for minutes at a time without shooting (they attempted 13 shots for the game) to limit the impact of the Lakers' dominant George Mikan. It led the St. Paul Dispatch to write, "[The Pistons] gave pro basketball a great black eye." :31–2 NBA President Maurice Podoloff said, "In our game, with the number of stars we have, we of necessity run up big scores." :33 A few weeks after the Pistons/Lakers game, the Rochester Royals and Indianapolis Olympians played a six-overtime game with only one shot in each overtime: in each overtime period, the team that had the ball first held it for the entirety of the period before attempting a last-second shot. The NBA tried several rule changes in the early 1950s to speed up the game and reduce fouls before eventually adopting the shot clock.
In 1954 in Syracuse, New York, Syracuse Nationals (now the Philadelphia 76ers) owner Danny Biasone and general manager Leo Ferris experimented with a 24-second shot clock during a scrimmage.   Jack Andrews, longtime basketball writer for The Syracuse Post-Standard, often recalled how Ferris would sit at Danny Biasone's Eastwood bowling alley, scribbling potential shot clock formulas onto a napkin.  According to Biasone, "I looked at the box scores from the games I enjoyed, games where they didn't screw around and stall. I noticed each team took about 60 shots. That meant 120 shots per game. So I took 2,880 seconds (48 minutes) and divided that by 120 shots. The result was 24 seconds per shot."   :29 Ferris was singled out by business manager Bob Sexton at the 1954 team banquet for pushing the shot clock rule. Biasone and Ferris then convinced the NBA to adopt it for the 1954–55 season, a season in which the Nationals won the NBA Championship.
When it was introduced by the NBA, the 24-second shot clock made players so nervous that it hardly came into play, as players were taking fewer than 20 seconds to shoot. According to Syracuse star Dolph Schayes, "We thought we had to take quick shots – a pass and a shot was it – maybe 8–10 seconds... But as the game went on, we saw the inherent genius in Danny's 24 seconds – you could work the ball around [the offensive zone] for a good shot." :29
The shot clock, together with some rule changes concerning fouls, revolutionized NBA basketball. In the last pre-clock season (1953–54), teams averaged 79 points per game; in the first year with the clock (1954–55), the average was 93 points,  which went up to 107 points by its fourth year in use (1957–58). :28 The advent of the shot clock (and the resulting increase in scoring) coincided with an increase in attendance, which increased 40% within a few years to an average of 4,800 per game. :33–4
The shot clock received near-universal praise for its role in improving the style of play in the NBA. Coach and referee Charley Eckman said, "Danny Biasone saved the NBA with the 24-second rule."  Boston Celtic all-star Bob Cousy said, "Before the new rule, the last quarter could be deadly. The team in front would hold the ball indefinitely, and the only way you could get it was by fouling somebody. In the meantime, nobody dared take a shot and the whole game slowed up. With the clock, we have constant action. I think it saved the NBA at that time. It allowed the game to breathe and progress."  League president Maurice Podoloff called the adoption of the shot clock "the most important event in the NBA."  The league itself states, "Biasone's invention rescue[d] the league." 
Two later pro leagues that rivaled the NBA adopted a modified version of the shot clock. The American Basketball League used a 30-second shot clock for its two years in existence (1961–1963). The American Basketball Association also adopted a 30-second clock when it launched in 1967–68, switching to the NBA's 24-second length for its final season (1975–76).
In the 1969–70 season, women's collegiate basketball (at the time sanctioned by the Commission on Intercollegiate Athletics for Women) used a 30-second shot clock on an experimental basis, officially adopting it for the 1970–71 season.  Unlike the women's side, there was initial resistance to the implementation of a shot clock for men's NCAA basketball, due to fears that smaller colleges would be unable to compete with powerhouses in a running game. However, after extreme results like an 11–6 Tennessee win over Temple in 1973, support for a men's shot clock began to build.  The NCAA introduced the 45-second shot for the 1985–86 season;  several conferences had experimented with it for the two seasons prior.  It was reduced to 35 seconds in the 1993–94 season,  and 30 seconds in the 2015–16 season.  The NAIA also reduced the shot clock to 30 seconds for men's basketball starting in 2015–16. 
From its inception in 1975, the Philippine Basketball Association adopted a 25-second shot clock. This was because the shot clocks then installed at the league's main venues, the Araneta Coliseum and Rizal Memorial Coliseum (the latter no longer used by the league), could only be set at 5-second intervals. The league later adopted a 24-second clock starting from the 1995 season. The Metropolitan Basketball Association in the Philippines used the 23-second clock from its maiden season in 1998. In Philippine college basketball, the NCAA Basketball Championship (Philippines) and the UAAP Basketball Championship adopted a 30-second clock, then switched to 24 seconds starting with the 2001–02 UAAP season 64, the first season to start after the FIBA rule change in 2001.
The shot clock is reset to a certain number of seconds (usually 24) when the ball touches the rim or goes into the basket; and when possession of the ball switches to the other team, such as with a rebound, steal, or violation. The initial setting varies by country, level of play, and league; see the table below. In some cases, such as when the new offensive team does not have to travel the entire length of the court, the initial setting may be lower. The shot clock does not start to count down until a player achieves control of the ball, or in the case of a made basket, a player achieves control of the in-bounds pass.
The offensive team must shoot the ball before the shot clock expires. If the shot clock expires before the ball leaves the player's hand, the team has committed a shot clock violation that results in a turnover to their opponents. The buzzer may sound after the ball leaves the shooter's hand; this is not a violation.
Near the end of a period, if the shot clock would ordinarily display more time than the game clock, the shot clock is switched off (not to 0.0). The game clock shows how much time players have to shoot, because a shot must leave the hand before the end of the period.
The shot clock operator sits at the scorer's table. This is usually a different person from the scoreboard operator, as the task requires concentration during and after the shot attempt. In the 2016-17 NBA season, a new 'official timekeeper' deal for the NBA with Swiss watch manufacturer Tissot introduced technology to unify the keeping of the shot clock and the game clock.  Tissot also became official timekeeper for the WNBA in the 2017 season.
If the offensive team is fouled and the penalty does not include free throws but just an in-bounds pass, the shot clock is reset. There are several cases where the offense does not need a full 24 seconds. The shot clock is instead set to 14 following an offensive rebound. :7-IV-d The NBA adopted this in 1998 and FIBA adopted it in 2010. The WNBA also observes this rule.
In several other cases where the offense inbounds the ball in its frontcourt (such as a foul by the defense not resulting in free throws), the offense is guaranteed 14 seconds. :7-IV-e The shot clock is increased to 14 if it showed a shorter time.
- If the defensive team acquires possession, the shot clock is reset, as it is on any other change of possession.
- If the offense retains possession, the shot clock is not reset, because there was no change of possession. However, in Euroleague and the NBA, the shot clock is topped up to 14 seconds, as described above for a frontcourt inbounds pass.
American college basketball uses a 30-second shot clock, while Canadian university basketball uses a 24-second clock. The American women's game has used a 30-second clock since the 1970s, but the men's game did not adopt a shot clock until 1985. The men's limit was originally 45 seconds, and was shortened to 35 seconds in 1993 before going to 30 seconds in 2015.
The National Federation of State High School Associations (NFHS), which sets rules for high school basketball in the U.S., does not mandate the use of a shot clock, instead leaving the choice to use a clock and its duration up to each individual state association. Proposals to adopt a national shot clock for high school basketball have been voted down by the NFHS as recently as 2011. 
Nine U.S. states require the use of a shot clock of 30 or 35 seconds in high school competition: California, Maryland, Massachusetts, Michigan, New York, North Dakota, Rhode Island, South Dakota, and Washington.  The District of Columbia also uses a 30-second shot clock for public school (DCIAA) competition and for the DCSAA State Tournament, where public, private, and charter schools compete for the championship of the District of Columbia.
|NBA||24 seconds (but see 14-second clock above)|
|U Sports (Canadian universities)|
|NCAA, NAIA, USCAA, et al.||30 seconds|
|United States high school basketball||30/35 seconds (some states only)|
|FIBA||24 seconds (but see
14-second clock above)|
12 seconds ( 3x3 half-court) 
|NCAA Men's||80 seconds |
|NCAA Women's||90 seconds |
|Water polo||FINA||30 seconds|
|Canoe polo||ICF||60 seconds|
|Ten-pin bowling||PBA||25 seconds|
|Snooker||Snooker Shoot-Out||15 seconds (first five minutes)|
10 seconds (last five minutes)
( Three-cushion billiards)
(3 time-outs (40 sec.) possible)
A related rule to speed up play is that the offensive team has a limited time to advance the ball across the half-court line (the "time line"). Failure to do so is a "backcourt violation" resulting in a turnover to the other team. This rule was introduced in 1933, predating the shot clock by over 2 decades.
In men's college basketball, the interval is 10 seconds. FIBA and the NBA specified 10 seconds, but adopted an 8-second limit in 2000 and 2001, respectively. The violation may be referred to using this limit: a "10-second violation" or "8-second violation".
Generally, the time limit is not marked off by the shot clock; a referee counts the seconds through a visible motion of his hand or arm. However, women's college basketball introduced the 10-second limit in 2013–14, and provided that officials will not count the ten seconds but "will use the shot clock to determine if a 10-second violation has occurred." The referee calls a violation if the offense still has the ball in the backcourt when the shot clock has counted down from 30 to 20 and now shows 19 (which first occurs at 19.9 seconds left). 
Similarly, in field lacrosse a team has 20 seconds to get the ball across the midfield line any way it can, and then 10 seconds to get the ball into its opponents' goal box. To satisfy the latter limit it is not necessary that a player in possession of the ball enter the box; although that is the most common way of doing so, that count is ended if the ball simply touches the ground inside the box. Officials typically use a timer for the 20-second count, as they may also at the same time be counting the four seconds a defensive player is allowed to stay in the goal crease with the ball, and count off the ten seconds manually.
- Pitch clock, used in baseball
- Play clock, used in American and Canadian football.
- Four corners offense, offensive stall strategy in basketball
- Stall Count, used in the sport of Ultimate.
- Time limit, the same concept applied to video games.
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