20 C.F.R. § 404.406
Under the provisions described in § 404.403 , beginning with the month in which a person files an application and becomes entitled to benefits on an insured individual's earnings record, the benefit rate of other persons entitled on the same earnings record (aside from the individual on whose earnings record entitlement is based) are adjusted downward, if necessary, so that the maximum benefits payable on one earnings record will not be exceeded. An application may also be effective (retroactively) for benefits for months before the month of filing (see § 404.603 ). For any month before the month of filing, however, benefits that have been previously certified by the Administration for payment to other persons (on the same earnings record) are not changed. Rather, the benefit payment of the person filing the application in the later month is reduced for each month of the retroactive period to the extent that may be necessary, so that no earlier payment to some other person is made erroneous. This means that for each month of the retroactive period the amount payable to the person filing the later application is the difference, if any, between (a) the total amount of benefits actually certified for payment to other persons for that month, and (b) the maximum amount of benefits payable for that month to all persons, including the person filing later.
20 C.F.R. §404.406