Here are the basics:

• Portability applies to a surviving spouse. It allows a surviving spouse to use a deceased spouse's unused estate and gift tax exclusion, shielding more money from estate taxes, beyond the estate tax threshold of $5.49 million per person in 2017 ($11.58 million per person in 2020).
• Portability rules come with caveats. Individuals and advisers need to be aware that they must act quickly after the first spouse dies. That's because estate tax returns must be filed usually within nine months of the death to take advantage of portability.
• An executor makes an election on Form 706. This will allow the surviving spouse to pick up the unused portion of the partner's gift and estate tax exemption.
• Portability generally doesn't apply to state estate taxes. Many states have state estate taxes, and there is usually no portability provision at the state level. State estate tax levels may also be different from federal estate tax levels.
• Portability doesn't help control bequests. Portability doesn't control how assets will be distributed to heirs. So control is a more important estate planning objective. Whether or not portability is an issue, families should consider trusts, for example, as useful tools for handing money to future generations.
• Portability doesn't address asset appreciation. When the surviving spouse dies, only the original deceased spouse's estate would be protected, not any appreciation in the value of assets.

IRS regulations allow the value of assets going to the surviving spouse or charity to be estimated rather than a more formal and detailed appraisal. The estate tax return must be completed fully in other respects.

What if a couple's combined assets are far less, say, well below $5 million? Many experts still recommend taking advantage of portability because assets can rise in value, especially if the second death could be decades away.

Not opting for portability can shortchange a survivor, as it allows spouses to leave assets to each other free of estate tax. People have to be willing and able to plan ahead. Also, portability rules allow surviving spouses to carry the unused exemption of the last deceased spouse amount into their next marriage, but if the new spouse later dies only the most recent deceased spouse's unused exemption is if available to the surviving spouse except as to gifts made by the surviving spouse while the most recent spouse was alive. Let's just say it's complicated.

Tailor your estate plan to your individual situation and discuss portability with your wealth and tax advisers, as well as your estate attorney, as the law allows spouses to leave assets to each other free of estate tax.