A ground rule double in baseball refers to a rule which awards two bases from the time of which the pitch is thrown, to all baserunners. This rule is awarded as a result of the ball being hit out of play under a condition that has been set by the ground rules at the field where the game is being played.
During major league matches, each field has their own ground rules. These are a set of rules that relate to the field itself as they are all independent entities. Some grounds have their own specific rules in relation to the ballpark which should be adhered to.