Facilities must have on hand at least one 18 x 24 cm and one 24 x 30 cm moving grid mechanism for each film-screen mammography x-ray unit in the facility. In addition, if the facility uses image receptor sizes other than the 18 x 24 and the 24 x 30 sizes, the facility must have moving grid mechanisms matched to each of these additional sizes. Each of these must be capable of properly functioning with their respective units. Note that a single, large grid assembly that is capable of accepting multiple sized cassettes is not sufficient to meet this requirement. The reason for not allowing the use of the single large grid assembly is the problem such an assembly causes when trying to adequately position small-breasted patients.