Wikipedia
Marital deduction is a type of tax law that allows a person to give assets to his or her spouse with reduced or no tax imposed upon the transfer. Some marital deduction laws even apply to transfers made postmortem. Spouses can transfer property between themselves tax free and ex-spouses can do that according to divorce decree. For US estate and gift tax purposes, there is no tax on transfers between spouses, whether during lifetime or at death. There is no limit on the amount that may be transferred. However, there are two important exceptions. The federal gift tax marital deduction is only available if the donee spouse (the person receiving the gift) is a U.S. citizen. However, the federal estate tax marital deduction is available for bequests at death to a surviving spouse even if not a US citizen. However, if the survivor is not a US citizen, the bequest must take the form of a specialized type of trust known as a Qualified Domestic Trust.