Accumulation and concentration rules regulate common situations in which there are multiple defects on the surface of an optical element, and clarify how they should be added up. For example, if one or more scratches are found with the maximum visibility allowed, to pass the test, the sum of the length of these scratches is limited to a quarter of the diameter of the element. The number of digs at the maximum permitted level is determined by dividing the measured clear aperture diameter (in millimeters) by 20, and rounding up. For example, for a clear aperture of 81 mm, 5 digs are allowed at the maximum level.