The rules of polo are written for the safety of both players and horses. Games are monitored by umpires. A whistle is blown when an infraction occurs, and penalties are awarded. Strategic plays in polo are based on the "line of the ball", an imaginary line that extends through the ball in the line of travel. This line traces the ball's path and extends past the ball along that trajectory. The line of the ball defines rules for players to approach the ball safely. The "line of the ball" changes each time the ball changes direction. The player who hits the ball generally has the right of way, and other players cannot cross the line of the ball in front of that player. As players approach the ball, they ride on either side of the line of the ball giving each access to the ball. A player can cross the line of the ball when it does not create a dangerous situation. Most infractions and penalties are related to players improperly crossing the line of the ball or the right of way. When a player has the line of the ball on his right, he has the right of way. A "ride-off" is when a player moves another player off the line of the ball by making shoulder-to-shoulder contact with the other players' horses.