Two teams of four compete on a level, rectangular grass field that measures 200 by 300 yd (182.88 by 274.32 m). Safety zones surround the playing field, and at either end goal posts stand 10 ft (3.05 m) high and 24 ft (7.32 m) apart. An indoor version is tailored to the dimensions of the various arenas in which it is played. The outdoor ball, weighing about 41/2 oz (.13 kg) and measuring not more than 31/4 in. (8.26 cm) in diameter, is made of wood, often willow root. Standard polo equipment includes a specially made brimmed helmet, a flexible-stemmed mallet some 4 ft (1.22 m) long, and the usual equestrian equipment.
An outdoor match is made up of eight periods (called chukkers), usually of 71/2 min each, though in some matches either the length or number of chukkers may be reduced. Play is directed toward hitting the ball through the opponents' goal. A mounted umpire metes out penalties—e.g., automatic goals, free shots on goal, and disqualification—for dangerous riding, carrying the ball, or illegal use of the mallet. The umpire starts each period and begins play after each goal by throwing the ball into a marked-off midfield area between the two lines of opposing players. A system of handicapping players promotes parity.
Polo ponies, actually standard-size horses of no particular breed, undergo a long, rigorous period of training to prepare them for the bruising requirements of the game. Because a typical polo match involves virtually nonstop action and many high-speed collisions of the horses, each player must maintain a "string" of expensive ponies so as to be able to change mounts several times during the course of a match. Thus, polo is a sport for the wealthy.