In every park, a hit that goes out of play (into the stands or out of the park) just as a home run, but on the bounce after touching the ground in fair territory, is an automatic double as specified in Major League Baseball rules 6.09(e) through 6.09(g). The same applies if a batted ball becomes lodged in, or rolls under, the outfield fence. Such hits are typically referred to as ground rule doubles, which, while not truly a ground rule, has come to be an almost universally understood misnomer and has entered into accepted usage.
Any baserunners ahead of the batter are also entitled to advance two bases on a ground rule double, based on their positions when the ball was pitched. This sometimes has the effect of denying a team a run, since a runner starting from first base would frequently come around to score on a normal double to the outfield, but in this case must stop at (or go back to) third base. This also means, that if a man is on second base, and a ground rule double is hit, he advances to score.
An automatic double can also be awarded if the ball does not leave the field of play but becomes otherwise unplayable. Examples include the ball getting stuck under the roll of tarpaulin used to cover the infield during rain delays, and disappearing into the ivy covering the outfield wall of Wrigley Field.
One additional rule covers the very rare situation where a fielder deliberately uses his cap or mask to play the ball, or if he throws his glove at a batted ball that he cannot reach on his own. This rule (7.05 sections (b) and (c)) awards the batter and any runners three bases on such a play. The ball remains in play and the batter may advance to home base at his peril. This rule very rarely, if ever in the modern game, comes into effect.
Originally, 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.
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