26 C.F.R. § 1.1313(c)-1
An adjustment in the case of the taxpayer with respect to whom the error was made may be authorized under section 1311 although the determination is made with respect to a different taxpayer, provided that such taxpayers stand in one of the relationships specified in section 1313(c). The concept of related taxpayer has application to all of the circumstances of adjustment specified in § 1.1312-1 through § 1.1312-5 if the related taxpayer is one described in section 1313(c); it has application to the circumstances of adjustment specified in § 1.1312-6 only if the related taxpayer is one described in section 1313(c)(7); it does not apply in the circumstances specified in § 1.1312-7 . If such relationship exists, it is not essential that the error involve a transaction made possible only by reason of the existence of the relationship. For example, if the error with respect to which an adjustment is sought under section 1311 grew out of an assignment of rents between taxpayer A and taxpayer B, who are partners, and the determination is with respect to taxpayer A, an adjustment with respect to taxpayer B may be permissible despite the fact that the assignment had nothing to do with the business of the partnership. The relationship need not exist throughout the entire taxable year with respect to which the error was made, but only at some time during that taxable year. For example, if a taxpayer on February 15 assigns to his fiancee the net rents of a building which the taxpayer owns, and the two are married before the end of the taxable year, an adjustment may be permissible if the determination relates to such rents despite the fact that they were not husband and wife at the time of the assignment. See § 1.1311(b)-3 for the requirement in certain cases that the relationship exist at the time an inconsistent position is first maintained.
26 C.F.R. §1.1313(c)-1