Each team consists of equal numbers of players (excluding substitutes). There is no upper limit to the number of players, although it is most common to field eleven players in inter-house and other organised matches. Competition rules may state a minimum number of players required to constitute a team (this is usually seven). There are a variety of positions in which the players are strategically placed, though these positions are not defined or required by the Laws. The positions are usually the backs also referred to as the threes on account of their number, the forwards and the wings. There are two wings on each wing to facilitate the giving of "yards". The backs pivot on the centre back depending on the position of the ball - hence the graceful alignment referred to in the Harrow Song "Yards". The forwards are often four in number and are the powerhouse which clears the way to allow the ball to be taken forward.