Ground rule double

A ground rule double is a baseball rule that awards 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, such as when the ball's first bounce was within the field. The automatic double (or rule-book double) is commonly called a ground rule double.