Here's an acronym you've probably never heard of: DSUE, pronounced D-Sue, it stands for deceased spouse's unused exemption, and it could be a crucial component of your estate plan.

Frequently, a plan relies on two key tax-saving provisions—the unlimited marital deduction and the unified estate and gift tax exemption. Under the marital deduction, a spouse normally doesn't have to pay estate or gift tax on any property transferred from a spouse. The estate and gift tax exemption covers transfers to your children or other non-spouses up to $5.49 million in 2017.

That means that a married couple together can transfer almost $11 million to others without a penny of tax liability. Even better, the exemption is "portable" between spouses—so when the estate of the spouse who dies first doesn't exhaust all of that person's exemption, it can be used by the estate of the second spouse.