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. There are 18 positions in Australian rules football, not including four (sometimes 6–8) interchange players who may replace another player on the ground at any time during play. The fluid nature of the modern game means the positions in football are not as formally defined as in sports such as rugby or American football. Even so, most players will play in a limited range of positions throughout their career, as each position requires a particular set of skills. Footballers who are able to play comfortably in numerous positions are referred to as utility players. Back line The term back line can either refer to the full-back line consisting of the two back pockets and the full-back, or both the full- and half-back lines, which collectively can also be referred to as the defence, defensive unit or the back six. Full-back The full-back position has always been a purely defensive role, with the aim of stopping the full-forward from getting the ball and scoring. But, in recent times, where the ability to move the ball out of the back and down the field quickly has become a more important tactic, the full-back often starts a chain of passes up the ground. The defensive aspect of the position remains important, with the ability to accelerate and change direction quickly. Spoiling the ball is also of utmost importance. The full-back often kicks the ball back into play after a point has been scored, although some teams prefer a midfielder or the small back pockets for this role, freeing the (typically taller) fullback player to attempt to mark the kick in. Notable full-backs: George 'Jocka' Todd Jack Regan Vic Thorp Fred Baring Fred Hughson Stephen Silvagni Chris Langford Geoff Southby David Dench Kelvin Moore Gary Malarkey Rod Carter Gary Pert Michael Brennan Ashley McIntosh Darren Gaspar Ben Rutten Simon Prestigiacomo Darren Glass Brian Lake Matthew Scarlett Dustin Fletcher Mal Michael Mick Martyn Shane Wakelin Tom Jonas Alex Rance Harris Andrews Tom Lonergan Zac Dawson Back pocket The