One of the old estate planning techniques was to make sure that each spouse had approximately the same amount of assets for separate property states. In this case, a spouse passing away owning assets would get a step-up in basis and would not be subject to a "double-tax" if the other spouse passed away first.
As an example, assume Bert and Ermine were married and Bert owned $10 million in assets and Ermine owned none. Ermine passed away first and owed no estate tax (assume $5 million exemption). Now, Bert passes away and his heirs owe $2 million of tax on his $10 million estate. If Bert had passed first, he could have given $5 million to his kids tax-free and $5 million to Ermine and when she passed, no estate tax would be due.
As a result of this "inequity", Congress several years ago put in place the "portability" election by estates. Under this procedure, the surviving spouse can elect to "port" over any unused exemption from the first spouse to pass away. In our example, Bert could have elected to port over Ermine's unused $5 million exemption and his heirs could have used it when he passed and no estate tax would be owed.
However, in order to take advantage of portability, an election must be made by the estate. This requires filing a Form 706 "Estate Tax Return" even if no tax is owed. However, many estates were not aware of this requirement and the IRS was bombarded with late filings of estate tax returns to simply elect portability. These late elections required a $10,000 filing fee and more work by the IRS.
The IRS just announced in Revenue Procedure 2017-34 that these estates will now be granted an automatic extension to January 2, 2018 to file these types of returns. This applies for any estate from 2011 forward (portability was not available before 2011). Therefore, your family has had anyone pass away that the unused exemption amount would have value for the surviving spouse, don't delay in making this filing. It can save the family a lot of taxes.