26 U.S.C. § 1501

An affiliated group of corporations shall, subject to the provisions of this chapter, have the privilege of making a consolidated return with respect to the income tax imposed by chapter 1 for the taxable year in lieu of separate returns. The making of a consolidated return shall be upon the condition that all corporations which at any time during the taxable year have been members of the affiliated group consent to all the consolidated return regulations prescribed under section 1502 prior to the last day prescribed by law for the filing of such return. The making of a consolidated return shall be considered as such consent. In the case of a corporation which is a member of the affiliated group for a fractional part of the year, the consolidated return shall include the income of such corporation for such part of the year as it is a member of the affiliated group.