Each team consists of a maximum of eleven players (excluding substitutes ), one of whom must be the goalkeeper . Competition rules may state a minimum number of players required to constitute a team; this is usually seven. Goalkeepers are the only players allowed to play the ball with their hands or arms, but they are only allowed to do so within the penalty area in front of their own goal. Though there are a variety of positions in which the outfield (non-goalkeeper) players are strategically placed by a coach, these positions are not defined or required by the Laws.
The basic equipment ( kit ) players are required to wear includes a shirt, shorts, socks, footwear and adequate shin guards. Players are forbidden to wear or use anything that is dangerous to themselves or another player (including jewelry or watches). The goalkeeper must wear clothing that is easily distinguishable from that worn by the other players and the match officials.
A number of players may be replaced by substitutes during the course of the game. The maximum number of substitutions permitted in most competitive international and domestic league games is three, though the number permitted may be varied in other leagues or in friendly matches. Common reasons for a substitution include injury, tiredness, ineffectiveness, a tactical switch, or as a defensive ploy to use up a little time at the end of a finely poised game. In standard adult matches, a player who has been substituted may not take further part in the match.
A game is officiated by a referee , who has "full authority to enforce the Laws of the Game in connection with the match to which he has been appointed" (Law 5), and whose decisions are final. The referee is assisted by two assistant referees . In many high-level games there is also a fourth official (and in the world cup a fifth official ), who assist(s) the referee and may replace another official should the need arise.
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; the referee does, however, make allowance for time lost through significant stoppages as described below. 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 commonly referred to as stoppage time or injury time , and 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, and 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.
In league competitions games may end in a draw, but in some knockout competitions if a game is tied at the end of regulation time it may go into extra time, which consists of two further 15-minute periods. If the score is still tied after extra time, some competitions allow the use of penalty shootouts (known officially in the Laws of the Game as "kicks from the penalty mark") to determine which team will progress to the next stage of the tournament. Goals scored during extra time periods count toward the final score of the game, but kicks from the penalty mark are only used to decide the team that progresses to the next part of the tournament (with goals scored in a penalty shootout not making up part of the final score).
Competitions held over two legs (in which each team plays at home once) may use the away goals rule to attempt to determine which team progresses in the event of equal aggregate scores. If the result is still equal following this calculation kicks from the penalty mark are usually required, though some competitions may require a tied game to be replayed.
