Ground rule double
A ground rule double is a double awarded by the umpire because a fair ball became unplayable according the the ground rules of the ballpark. The ground rules technically only cover ways in which the ball can become unplayable, such as becoming lodged in the ivy at Wrigley Field; the rulebook specifies that the award is always two bases. The only exception to the two base rule is in the now very uncommon case of overflow crowds placed in cordoned off sections of the playing field, in which case the managers may agree on other base awards.
"Ground rule double" is also colloquially used for doubles awarded because "a fair ball, touching the ground, bounds into the stands". This colloquial usage is technically incorrect as the ruling is applied equally in all ballparks and has nothing to do with "ground rules". Some commentators will use terms such as "bounce doubles", "rulebook doubles", or as Jon Miller prefers, "automatic doubles" in lieu of the misappropriation of "ground rule double". Until 1931, balls bouncing over the fence were counted as home runs, and this difference in playing rules is respected when tabulating historical statistics.
A ground rule double in either the actual or colloquial sense allows all runners to advance exactly two bases from when the play began. A runner from first base is thus required to stop at third, even if he obviously could have scored had the ball not gone out of play. This rigid awarding of bases distinguishes automatic doubles from fan interference, in which the umpire is free to award as many bases to each player as he deems the player would have attained without the interference.