An over-under or over/under (O/U) bet is a wager in which a sportsbook

Sportsbook

In the United States a sportsbook or a race and sports book is a place where a gambler can wager on various sports competitions, including golf, football, basketball, baseball, hockey, soccer, horse racing, boxing, and mixed martial arts. The method of betting varies with the sport and the type of...

 will predict a number for a statistic in a given game (usually the combined score of the two teams), and bettors wager that the actual number in the game will be either higher or lower than that number. For example, in Super Bowl XXXIX

Super Bowl XXXIX

Super Bowl XXXIX was an American football game played on February 6, 2005, at Alltel Stadium in Jacksonville, Florida, to decide the National Football League champion following the 2004 regular season...

, most Las Vegas

Las Vegas Strip

The Las Vegas Strip is an approximately stretch of Las Vegas Boulevard in Clark County, Nevada; adjacent to, but outside the city limits of Las Vegas proper. The Strip lies within the unincorporated townships of Paradise and Winchester...

 casino

Casino

In modern English, a casino is a facility which houses and accommodates certain types of gambling activities. Casinos are most commonly built near or combined with hotels, restaurants, retail shopping, cruise ships or other tourist attractions...

s set the over-under for the score of the game at 46.0. A bettor could wager that the combined score of the two teams would be either more than or less than that number. Since the combined score of that game was 45, anyone who had bet on "under" won.

Initial value

The goal of a sportsbook manager is to have an equal number of bets on both sides of the over-under. In theory, this means that the manager could set the value at zero and then readjust based on either the number of incoming bets and/or events that influence the potential outcome. In practice, the initial value is based on both quantitative (e.g. win-loss record, average points per game, etc) and anecdotal information (e.g. media reports, injury status of players, etc).

Statistics

Though this bet is most commonly made with the combined score of the two teams, many other statistics can be used, including:

• In American football
  American football
  American football is a sport played between two teams of eleven with the objective of scoring points by advancing the ball into the opposing team's end zone. Known in the United States simply as football, it may also be referred to informally as gridiron football. The ball can be advanced by...
  
  , a player's or team's total rush
  Rush (American football)
  Rushing has two different meanings in gridiron football .-Offense:The first is an action taken by the offensive team that means to advance the ball by running, as opposed to passing. A run is technically any play that does not involve a forward pass...
  
  ing yard
  Yard
  A yard is a unit of length in several different systems including English units, Imperial units and United States customary units. It is equal to 3 feet or 36 inches...
  
  s or attempts, down
  Down (football)
  A down is a period in which a play transpires in American and Canadian football.-Description:A down begins with a snap or free kick , and ends when the ball or the player in possession of it is declared down by an official, a team scores, or the ball or player in possession of it leaves the field...
  
   conversions (first or third), interceptions, completions, field goal percentage
  Field goal percentage
  Field goal percentage in basketball is the ratio of field goals made to field goals attempted. Its abbreviation is FG%. Three-point field goals are included in this percentage. Instead of using scales of 0 to 100%, the scale .000 to 1.000 is commonly used. A higher field goal percentage denotes...
  
  , etc.
• In basketball
  Basketball
  Basketball is a team sport in which two teams of five players try to score points by throwing or "shooting" a ball through the top of a basketball hoop while following a set of rules...
  
  , a player's or team's total assist
  Assist (basketball)
  In basketball, an assist is attributed to a player who passes the ball to a teammate in a way that leads to a score by field goal, meaning that he or she was "assisting" in the basket. There is some judgment involved in deciding whether a passer should be credited with an assist...
  
  s, block
  Block (basketball)
  In basketball, a block , not to be confused with blocking, occurs when a defensive player legally deflects a field goal attempt from an offensive player. The defender must not touch the offensive player's hands or otherwise a foul is called. In order to be legal, the block must occur while the shot...
  
  s, turnovers
  Turnover (basketball)
  In basketball, a turnover occurs when a player from one team gives possession to a member of the opposing team by losing the ball. This can result from the ball being stolen, the player making mistakes such as stepping out or throwing the ball out of bounds, or committing a violation or committing...
  
  , steal
  Steal
  Steal may refer to:* Theft, the illegal taking of another person's property without that person's freely-given consent* The gaining of a stolen base in baseball...
  
  s, etc.
• In baseball
  Baseball
  Baseball is a bat-and-ball sport played between two teams of nine players each. The aim is to score runs by hitting a thrown ball with a bat and touching a series of four bases arranged at the corners of a ninety-foot diamond...
  
  , a player's or team's total number of home run
  Home run
  In baseball, a home run is scored when the ball is hit in such a way that the batter is able to reach home safely in one play without any errors being committed by the defensive team in the process...
  
  s, RBI, etc.
• In Association Football, The amount of goals scored in a game. The most popular market being Under/Over 2.5 goals