This Article analyzes portability and its antecedents in order to distill a positive account of marital sharing of transfer tax exemption amounts. Prior to 2010, the estate and gift tax exemption equivalent was a non-transferable, separate tax attribute of each spouse. A spouse could only access his or her spouse’s effective exemption by shifting property into the other spouse’s tax base. With the enactment of portability, Congress decoupled tax-free availability of a spouse’s unified credit from the necessity of a prior intra-spousal transfer. All that is required is an election by the decedent spouse, via the executor, to share the decedent’s unused exemption equivalent with the surviving spouse. This Article argues that a logical extension of this progression in the law, presaged by several early proposals by the American Law Institute and the U.S. Treasury, would be a regime that authorized elective sharing of estate and gift tax exemption amounts between spouses, in any proportion, during life or at death.