26 C.F.R. § 1.544-7
Example. Two brothers, A and B, each own 10 percent of the stock of the M Corporation, and A's wife, AW, also owns 10 percent of the stock of such corporation. AW's husband, A, has an option to acquire the stock owned by her at any time. It becomes necessary, for one of the purposes stated in section 544(a)(4), to determine the stock ownership of B in the M Corporation. If the family and partnership rule were the only rule that applied in the case, B would be considered, under that rule, as owning 20 percent of the stock of the M Corporation, namely, his own stock plus the stock owned by his brother. In that event, B could not be considered as owning the stock held by AW since (1) AW is not a member of B's family and (2) the constructive ownership of such stock by A through the application of the family and partnership rule in his case is not considered as actual ownership so as to make B the constructive owner by a second application of the same rule with respect to the ownership of the stock. However, there is more than the family and partnership rule involved in this example. As the holder of an option upon the stock, A may be considered the constructive owner of his wife's stock by the application of the option rule and without reference to the family relationship between A and AW. If A is considered as owning the stock of his wife by application of the option rule, then such constructive ownership by A is regarded as actual ownership for the purpose of applying the family and partnership rule so as to make another member of A's family, for example, B, the constructive owner of the stock. Hence, since A may be considered as owning his wife's stock by applying either the family-partnership rule or the option rule, the provisions of section 544(a)(6) apply and accordingly A must be considered the constructive owner of his wife's stock under the option rule rather than the family-partnership rule. B thus becomes the constructive owner of 30 percent of the stock of the M corporation, namely, his own 10 percent, A's 10 percent, and AW's 10 percent constructively owned by A as the holder of an option on the stock.
26 C.F.R. §1.544-7