and levels of play and players are organized within and between polo clubs, including membership, rules, safety, fields and arenas. The rules of polo are written to include 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 their right, they have 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. The defending player has a variety of opportunities for their team to gain possession of the ball. They can push the opponent off the line or steal the ball from the opponent. Another common defensive play is called "hooking." While a player is taking a swing at the ball, their opponent can block the swing by using their mallet to hook the mallet of the player swinging at the ball. A player may hook only if they are on the side where the swing is being made or directly behind an opponent. A player may not purposely touch another player, another player's tack, or a pony with their mallet. Unsafe hooking is a foul that will result in a penalty shot being awarded. For example, it is a foul for a player to reach over an opponent's mount in an attempt to hook. The other basic defensive play is called the bump or ride-off. It's