A cricket field consists of a large grassy ground on which the game of cricket is played; this is ostensibly an oval though most cricket grounds would more accurately be described either as a convex set or as a convex polygon with an infinite number of sides. Cricket grounds can be almost perfectly circular, elongated ovals or entirely irregular shapes with little or no symmetry - but they will have entirely curved boundaries, almost without exception. There are no fixed dimensions for the field but its diameter usually varies between 450 feet (137 m) and 500 feet (150 m). Cricket is unusual among major sports (along with Golf, Australian rules football and baseball) in that there is no official rule for a fixed-shape ground for professional games. On most grounds, a rope demarcates the perimeter of the field and is known as the boundary. Within the boundary and generally as close to the centre as possible will be the square which is an area of carefully prepared grass upon which cricket pitches can be prepared and marked for matches.