An opening line is the initial odds released by a sportsbook for a given event. Betting limits start off low and rise gradually the longer the market is open. This allows oddsmakers to adjust their lines based on early sharp action. American football opening lines are posted a week in advance, while lines for other sports are released the night before. Opening lines originate at high limit offshore betting sites such as BetCRIS. These lines are then cloned by other sportsbooks including those with physical locations in Las Vegas, Nevada.
Experienced sports bettors consider the opening line to be the time when the market is at it's lowest expected efficiency. There exists a larger edge due to discrepancies between the betting line and the market's expected value. Oddsmakers take advantage of early bets placed by sharp players in order to shape their lines to be more efficient. This is why it is not recommended to bet into closing lines.