The outfield wall or fence is the outer boundary of the outfield. A ball passing over the wall is dead. If it passes over the wall in fair territory, without touching the ground, it is a home run. The official rules do not specify the shape, height, or composition of the wall, or a specific mandatory distance from home plate (though Major League Baseball mandates a minimum distance of 250 feet (76 m) and recommends a minimum distance of 320 feet (98 m) at the foul poles and 400 feet (120 m) at center field). As a result, baseball fields can vary greatly along those lines. The wall has numbers affixed or painted on it that denote the distance from that point on the wall to home plate. In most modern major league ballparks, the wall is made of some hard material (e.g., concrete, plywood, sheet metal) with padding on the field side to protect players who may collide with the wall at high speed while trying to make a play. Chain link fencing may also be incorporated into the wall in areas where the wall needs to be transparent, e.g., an outfield bullpen, a spectator area behind the wall, or to protect a scoreboard incorporated into the wall. Many ballparks feature a yellow line denoting the top of the wall to aid umpires in judging whether the ball passed over the wall or if the ball is fair or foul.