Example: A, who makes his returns upon the calendar year basis, in 1935 purchased the X Building and subsequently gave it to his son B. B exchanged the X Building for the Y Building in a tax-free exchange, and then gave the Y Building to his wife C. C, in determining the gain from the sale or disposition of the Y Building in 1954, is required to reduce the basis of the building by deductions for depreciation which were successively allowed (but not less than the amount allowable) to A and B upon the X Building and to B upon the Y Building, in addition to the deductions for depreciation allowed (but not less than the amount allowable) to herself during her ownership of the Y Building.
26 C.F.R. §1.1016-10