20 CFR 222.22: Relationship as divorced spouse.

A claimant will be considered to be the divorced spouse of an employee if—

(a) His or her marriage to the employee has been terminated by a final divorce; and

(b) He or she is not married (if the claimant remarried after the divorce from the employee, the later marriage has been terminated by death, final divorce, or annulment); and

(c) He or she had been validly married to the employee, as set forth in § 222.11, for a period of 10 years immediately before the date the divorce became final. The claimant meets this requirement even if the claimant and employee were divorced within the ten-year period, provided that the claimant and employee were remarried no later than the calendar year immediately following the year in which the divorce took place.