Players are given a foul if they break the rules. These rules keep the horses and riders safe. One rule in polo is called the "line of the ball" or just the "line". The "line" is an invisible line between the ball and the player that has the ball. A player has the ball if they are the closest to make a play on the ball. The "line" is in the same direction as this player is traveling and extends as far as the player is able to safely stop their horse. If another player crosses the "line" (that is, if another player runs in front of the player who has the ball), they will receive a foul. This is a common foul because it is sometimes difficult for a player to tell where the invisible "line" is.