similar to a body check in hockey. In a ride-off, a player rides their pony alongside an opponent's mount to move an opponent away from the ball or to take them out of a play. It must be executed properly so that it does not endanger the horses or the players. The angle of contact must be safe and can not knock the horses off balance, or harm the horses in any way. Two players following the line of the ball and riding one another off have the right of way over a single man coming from any direction. Like in hockey or basketball, fouls are potentially dangerous plays that infringe on the rules of the game. To the novice spectator, fouls may be difficult to discern. There are degrees of dangerous and unfair play and penalty shots are awarded depending based on the severity of the foul and where the foul was committed on the polo field. White lines on the polo field indicate where the mid-field, sixty, forty and thirty yard penalties are taken. The official set of rules and rules interpretations are reviewed and published annually by each country's polo association. Most of the smaller associations follow the rules of the Hurlingham Polo Association, the national governing body of the sport of polo in the United Kingdom, and the United States Polo Association. Outdoor polo Outdoor or field polo lasts about one and a half to two hours and consists of four to eight seven-minute chukkas, between or during which players change mounts. At the end of each seven-minute chukka, play continues for an additional 30 seconds or until a stoppage in play, whichever comes first. There is a four-minute interval between chukkas and a ten-minute halftime. Play is continuous and is only stopped for rule infractions (fouls), broken tack (equipment) or injury to horse or player. The object is to score goals by hitting the ball between the goal posts, no matter how high in the air. If the ball goes wide of the goal, the defending team is allowed a free 'knock-in' from the place where the ball crossed the goal line, thus getting ball back into play. Indoor or arena polo Arena polo has rules similar to the field version, and is less strenuous for the player. It is played in a enclosed arena, much like those used for other equestrian sports; the minimum