 What is a ground rule double? Well to get technical there is no such thing. What most people are referring to when they say ground rule double is not actually a ground rule but an actual rule rule like in the rule book. In this case it's rule 5.05a6 which says that if a fair ball bounces off of the ground and then goes over the fence or through the fence or under the fence or through or under a scoreboard or through or under shrubbery or vines on the fence then the batter and all the runners are entitled to advance two bases which is a double. It's also a double if the ball gets stuck in any shrubbery vines or scoreboards on the wall whether the ball bounces first or not. The ground rules that do apply to all major league ballparks do mention doubles though specifically that if a fly ball hits the face of a lower wall and then bounces over a taller wall then the batter and runners are to be awarded two bases. It's a fairly unlikely occurrence from a physics perspective but it is a true ground rule double nonetheless. If the order is reversed however and a fly ball hits a taller wall at a point higher than an adjacent lower wall and deflects over that lower wall then it is a ground rule home run.