Polo is a team sport played on horseback in which the objective is to score goals against an opposing team. Players score by driving a small white plastic or wooden ball into the opposing team's goal using a long-handled mallet. The traditional sport of polo is played at speed on a large grass field up to 300 yards long by 160 yards wide, and each polo team consists of four riders and their mounts. Field polo is played with a solid plastic ball, which has replaced the wooden ball in much of the sport. In arena polo, only three players are required per team and the game usually involves more maneuvering and shorter plays at lower speeds due to space limitations of the arena. Arena polo is played with a small air-filled ball, similar to a small soccer ball. The modern game lasts roughly two hours and is divided into periods called chukkers. Polo is played professionally in 16 countries. It was formerly, but is not currently, an Olympic sport.