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In baseball, a ground rule double is an award of two bases from the time of pitch to all baserunners including the batter-runner as a result of the ball leaving play after being hit fairly and leaving the field under a condition of the ground rules in effect at the field where the game is being played. An automatic double is the term used to refer to a fairly hit ball leaving the field in circumstances that do not merit a home run as described in Major League Baseball (MLB) rules 6.09(e) through 6.09(h). The automatic double is quite often mistakenly called a ground rule double.
Most commonly, a ground rule double results from a batted ball hitting the ground in fair territory and landing out of play due to some unique aspect of the grounds, typically by bouncing over a fence or wall in the outfield. MLB rules also award an automatic double when a batted ball goes through or under a fence or through or sticking in shrubbery or vines on the fence. Specific rules also govern when fair fly balls are deflected into the stands by a fielder: for example, a fair fly ball deflected out of play by a fielder from a point within 250 feet of home plate is considered a double. This applied in an unusual play August 3, 2007 when Melky Cabrera of the New York Yankees hit a ball that ricocheted off Kansas City Royals pitcher Ryan Braun's foot and bounced into the stands in foul territory.
When two bases are awarded by either ground rule or league-wide rule, any baserunners ahead of the batter are entitled to advance two bases from their positions at the time of pitch but may advance further if the umpire feels the batter/runner would have made it to the third base completely uncontested. (7.0 MLB Umpire Handbook)
Previously, all batted balls that cleared the fence after a bounce in fair territory or on a fly were counted as home runs. The rule was changed by the American League prior to the 1930 season and was subsequently adopted by the National League on December 12, 1930.