In association football, the formation describes how the players in a team generally position themselves on the pitch. Association football is a fluid and fast-moving game, and (with the exception of the goalkeeper) a player's position in a formation does not define their role as rigidly as for, for instance, a rugby player, nor are there episodes in play where players must expressly line up in formation (as in gridiron football). Nevertheless, a player's position in a formation generally defines whether a player has a mostly defensive or attacking role, and whether they tend to play towards one side of the pitch or centrally.