The balls are not quite round. There's a slight bulge on one side, called the bias. When a ball is moving rapidly, it travels in a straight line, but as it slows down it begins to roll in the direction of the bias. A bowler can therefore make the ball curve in one direction or the other by positioning the bias during delivery. The duration of the game is decided beforehand.
Teams usually play a certain number of ends.
Each bowler gets two balls per end, no matter how many bowlers are involved. A variation, crown green bowling, is popular in the northwest of England. It's played on a rectangular green that slopes gently to a crown, about 14 inches high, in the center. The entire green can be used. Players may bowl from one side or one corner of the green to its opposite.