A fundamental of estate planning has to do with the portability of the estate tax exemption. This means that if the first spouse dies and doesn't use all of his or her federal exemption for estate taxes that the surviving spouse can add the unused estate tax exemption of the deceased spouse to their own exemption, allowing spouses together to transfer up to $10 million tax free.
However, it is important to note that portability is not automatic. To get it, the executor of the first spouse who died must file an estate tax return, even if no estate taxes are due. The safest course for most people is to review their situation with an attorney to make sure portability options are maximized.