26 C.F.R. § 25.2523(d)-1
Section 2523(d) provides that if a property interest is transferred to the donee spouse as sole joint tenant with the donor or as a tenant by the entirety, the interest of the donor in the property which exists solely by reason of the possibility that the donor may survive the donee spouse, or that there may occur a severance of the tenancy, is not for the purposes of section 2523(b), to be considered as an interest retained by the donor in himself. Under this provision, the fact that the donor may, as surviving tenant, possess or enjoy the property after the termination of the interest transferred to the donee spouse does not preclude the allowance of the marital deduction with respect to the latter interest. Thus, if the donor purchased real property in the name of the donor and the donor's spouse as tenants by the entirety or as joint tenants with rights of survivorship, a marital deduction is allowable with respect to the value of the interest of the donee spouse in the property (subject to the limitations set forth in § 25.2523(a)-1 ). See paragraph (c) of § 25.2523(b)-1 , and section 2524.
26 C.F.R. §25.2523(d)-1