In the sport of Australian rules football, each of the eighteen players in a team is assigned to a particular named position on the field of play. These positions describe both the player's main role and by implication their location on the ground. As the game has evolved, tactics and team formations have changed, and the names of the positions and the duties involved have evolved too. In total there are 18 positions in Australian rules football, not including 4 (sometimes 6 - 8) interchange players who may come onto the ground at any time during play to replace another player.