Each quarter has a specific length of playing time, which can vary in different forms of the game, but at senior level is usually 20 minutes.
When the umpire stops play for a score, injury, the blood rule, to award a 50-metre penalty or to reset play for a mark or free kick, he raises one hand above his head and blows his whistle; this is called blowing time off.
Time on was first introduced to the Laws of the Game by the Australian National Football Council for the 1928 season.
From 1994, the AFL Commission adopted the shorter 20 minute quarter, and introduced time-on for many other stoppages, including a ball-up or boundary throw-in.
The coaches' boxes and television networks are provided with a feed to the timekeeper's count-down clock in professional games.