20 CFR §404.251

1. (a)Disability before 1979; second entitlement after 1978. In this situation, we compute your second-entitlement primary insurance amount by selecting the highest of the following:
   1. (1)The primary insurance amount to which you were entitled when you last received a benefit, increased by any automatic cost-of-living or ad hoc increases in primary insurance amounts that took effect since then;
   2. (2)The primary insurance amount resulting from a recomputation of your primary insurance amount, if one is possible; or
   3. (3)The primary insurance amount computed for you as of the time of your second entitlement under any method for which you are qualified at that time, including the average-indexed-monthly-earnings method if the previous period of disability is disregarded.
2. (b)Disability and second entitlement after 1978. In this situation, we compute your second-entitlement primary insurance amount by selecting the highest of the following:
   1. (1)The primary insurance amount to which you were entitled when you last received a benefit, increased by any automatic cost-of-living or ad hoc increases in primary insurance amount that took effect since then;
   2. (2)The primary insurance amount resulting from a recomputation of your primary insurance amount, if one is possible (this recomputation may be under the average-indexed-monthly-earnings method only); or
   3. (3)The primary insurance amount computed for you as of the time of your second entitlement under any method (including an old-start method) for which you are qualified at that time.
3. (c)Disability before 1986; second entitlement after 1985. When applying the rule in paragraph (b)(3) of this section, we must consider your receipt of a monthly pension based on noncovered employment. (See § 404.213). However, we will disregard your monthly pension if you were previously entitled to disability benefits before 1986 and in any of the 12 months before your second entitlement.