A standard adult football match consists of two periods of 45 minutes each, known as halves. Each half runs continuously, meaning that the clock is not stopped when the ball is out of play. There is usually a 15-minute half-time break between halves. The end of the match is known as full-time. The referee is the official timekeeper for the match, and may make an allowance for time lost through substitutions, injured players requiring attention, or other stoppages. This added time is most commonly referred to as stoppage time or injury time, while loss time can also be used as a synonym. The duration of stoppage time is at the sole discretion of the referee. The referee alone signals the end of the match. In matches where a fourth official is appointed, toward the end of the half the referee signals how many minutes of stoppage time he intends to add. The fourth official then informs the players and spectators by holding up a board showing this number. The signalled stoppage time may be further extended by the referee. Added time was introduced because of an incident which happened in 1891 during a match between Stoke and Aston Villa. Trailing 1–0 and with just two minutes remaining, Stoke were awarded a penalty. Villa's goalkeeper kicked the ball out of the ground, and by the time the ball had been recovered, the 90 minutes had elapsed and the game was over. The same law also stands that the duration of either half is extended until the penalty kick to be taken or retaken is completed, thus no game shall end with a penalty to be taken.