Play begins with a throw in of the ball by the umpire at the opening of each chukker and after each goal. Players must change horses after each chukker due to the extreme demands placed on the polo pony. During half time, spectators go onto the field to participate in a tradition called divot stomping to help replace the divots created by the horses hooves. Polo players are ranked yearly by their peers and the USPA on a scale of 2 to 10 goals. Team play is handicapped on the basis of ability. Most of the rules of polo are for the safety of the polo players and their ponies. The basic concept is the line of the ball, a right of way established by the path of a traveling ball. Two mounted Umpires do most of the officiating, with a Referee at midfield having the final say in any dispute between the umpires.