25.2513-4—Joint and several liability for tax.
         If consent to the application of the provisions of  section 2513 is signified as provided in  § 25.2513-2, and not revoked as provided in  § 25.2513-3, the liability with respect to the entire gift tax of each spouse for such “calendar period” (as defined in  § 25.2502-1(c)(1)) is joint and several. See paragraph (d) of  § 25.2511-1.