The game consists of various numbers of chukkas, or periods, of seven and a half minutes. Players score by using a long handled mallet to hit a ball into a goal while on horseback. Polo is traditionally played on a 300 yard by 160 yard grass field with three or four players on each team. There are many rules in polo designed to keep the players and horses safe, the most governing rule being the line of the ball. To put simply, the line of the ball is an imaginary pencil thin line that travels all the way around the world depending on the direction the ball is traveling. This line is like lanes highway, you can not cross into another lane. The horses used in polo are most commonly Thoroughbreds and Argentine Thoroughbreds ( an American Thoroughbred crossed with the Argentine Criollo) . The manes of polo ponies are clipped off to ensure that the riders hand does not become tangled in the mane while playing. The horses tails are also braided and tied up with tape to prevent the mallet from getting caught.