This diagram shows the sizes and markings of an American football field, which gave the game the nickname ‘gridiron’. The field is divided into gradations of 10 yards (9.14 metres) because the game progresses by each team attempting to gain at least 10 yards in four plays or 'downs'. If they succeed, they get another four downs to advance again; if they fail the other team gains possession of the ball. The exact yardage is crucial and often the officials come onto the ground with a 10-yard measuring chain. At each end, in the middle of the goal line, is a goal post. Following a touchdown (worth six points) a team has an opportunity to gain an extra point by kicking the ball over the crossbar and between the uprights of the goal post. They can also attempt this from the field as one of their downs, and if the ball goes over the crossbar the team scores three points. Unlike rugby goal posts, those in American football stand on one upright which is positioned in the centre of the crossbar. Because play involves shifting between a team's defensive and offensive units, depending on which team has possession of the ball, the field includes a large area behind each sideline in which the players who are not on the field await their turn.