Page "Association football" Paragraph 9

One of the attractions of association football is that a casual game can be played with only minimal equipment – a basic game can be played on almost any open area of reasonable size with just a ball and items to mark the positions of two sets of goalposts.

Such games can often have team sizes that vary considerably from 11-a-side, use a limited and / or modified subset of the official rules, and are likely to be self-officiated by the players.