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Five-a-side football is a variation of association football in which each team fields five players (four outfield players and a goalkeeper). Other differences from football include a smaller pitch, smaller goals, a reduced game duration. Matches are played indoors, or outdoors on AstroTurf or artificial grass pitches that may be enclosed within a barrier or "cage" to prevent the ball from leaving the playing area and keep the game flowing.
The penalty area is significantly different from football: it is semi-circular in shape, only the goalkeeper is allowed to touch the ball within it, and he or she may or may not be allowed out. Goalkeepers are only allowed to give the ball out to another player through hands. The goalkeeper may only kick the ball if it is in the course of making a save. There are no offside rules. Headers are allowed. No protocol of deliberate handball vs accidental handball - referee needs to make a decision based on the distance from where the ball was hit. Yellow cards may result in the offending player being sent to the "sin bin" for a predetermined length of time. Red cards work in the same way as the 11-a-side game. Charging/sliding tackles are awarded a yellow card.
Additionally, metal studded boots or blades cannot be worn, as it damages the playing surface. Players are also required to wear shin guards but this is usually at the discretion of the referee.
Five-a-side is commonly played informally, and the rules are therefore flexible and are sometimes decided immediately before play begins; this is in contrast to futsal, for which official laws are published by FIFA.
The English FA have drawn up a full list of laws for the small-sided game which expands upon the rules outlined above and includes minimum/maximum pitch dimensions as well as technicalities on free-kicks and other parts of the game.
Futsal is a version of indoor five-a-side football developed by the Asociación Mundial de Fútbol de Salón (AMF / English:World Futsal Association). It currently has two governing bodies: Asociación Mundial de Fútbol de Salón, and the Fédération Internationale de Football Association (FIFA).
Beach Soccer is a variation on five-a-side football in that it is played on a sandy surface. Rules do not greatly differ from those found in regular five-a-side football.
A variation with increased pitch size and number of players on a team. In this variation there are five outfield players and one goalkeeper on the pitch for each team at any one time. Rules do not greatly differ from those found in five-a-side football.
This is another variation with increased pitch and team size; in this case with six outfield players and a goalkeeper on each side. The rules generally do not differ from those from five-a-side.
This is a variation of seven-a-side football primarily played in Australia and New Zealand. The rules have been modified slightly to encourage new players to the game, with strict enforcement of non-contact and two ways to score points: by scoring a goal in the same manner as the other formats, or by scoring a board that is on either side of the goal. The boards are usually 2.5m long and one third of the height of the goal. A goal is 3 points and a board is 1 point. When the ball goes out of play, it may be kicked or thrown in. This applies to the sideline and corners.
The popularity of five-a-side youth football has grown tremendously within the US.   Many organisations have chosen this format and modified it slightly to promote an environment where children can excel early in youth sports.  American Youth Soccer Organization, United States Youth Soccer Association, and Fun Fair Positive Soccer are among the largest organisations bringing this format to the regional US-based youth soccer arena.      
In recent years a few five-a-side teams have found themselves with sponsorship deals amounting up to thousand of pounds contracts. Sponsors feel with the vast numbers of participation in five-a-side football rising in the UK that it is a good place to advertise and tend to sponsor competition winners or league winners at local facilities so that they know that their deals are with the best five-a-side teams around the area.
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