The ball used in soccer matches should be spherical, its shell made of leather or other permitted materials, the quality of the ball material being decided by the International Football Association Board at any time. Any permitted material must be approved by the International Football Board, and materials that could harm players must not be used in the ball's structure. The circumference of the ball must not exceed 71 cm or be less than 68 cm. The weight of the ball must not exceed 453 grams or be less than 396 grams at the start of the match. After inflation, the pressure should be equivalent to 0.6 to 1.1 atmospheres (at sea level), which is equal to 600 to 1100 grams/cm². During the match, the ball may not be changed without the referee's permission.