play. However, a typical on-field structure consists of six forwards, six defenders or "backmen" and six midfielders, usually two wingmen, one centre and three followers, including a ruckman, ruck-rover and rover. Only four players from each team are allowed within the centre square () at every centre bounce, which occurs at the commencement of each quarter, and to restart the game after a goal is scored. There are also other rules pertaining to allowed player positions during set plays (that is, after a mark or free kick) and during kick-ins following the scoring of a behind. Match duration A game consists of four quarters and a timekeeper officiates their duration. At the professional level, each quarter consists of 20 minutes of play, with the clock being stopped for instances such as scores, the ball going out of bounds or at the umpire's discretion, e.g. for serious injury. Lower grades of competition might employ shorter quarters of play. The umpire signals time-off to stop the clock for various reasons, such as the player in possession being tackled into stagnant play. Time resumes when the umpire signals time-on or when the ball is brought into play. Stoppages cause quarters to extend approximately 5–10 minutes beyond the 20 minutes of play. 6 minutes of rest is allowed before the second and fourth quarters, and 20 minutes of rest is allowed at half-time. The official game clock is available only to the timekeeper(s), and is not displayed to the players, umpires or spectators. The only public knowledge of game time is when the timekeeper sounds a siren at the start and end of each quarter. Coaching staff may monitor the game time themselves and convey information to players via on-field trainers or substitute players. Broadcasters usually display an approximation of the official game time for television audiences, although some will now show the exact time remaining in a quarter. General play Games are officiated by umpires. Before the game, the winner of a coin toss determines which directions the teams will play to begin. Australian football begins after the first siren, when the umpire bounces the ball on the ground (or throws it into the air if the condition of the ground is poor), and the two ruckmen (typically the tallest players from each team) battle for the ball in the air on its way back down. This is known as the ball-up. Certain