If your spouse is a U.S. citizen, you can leave him or her an unlimited amount when you die with no estate tax. But there can be problems when the second spouse dies.

For example, let’s say Bob and Sue have a combined net estate of $10 million and they both die when the federal exemption is $5 million. When Bob dies, he leaves everything to Sue, so no estate taxes are due then. But when Sue dies, her estate of $10 million uses her $5 million exemption. The tax bill on the remaining $5 million? $1,750,000!

Congress tried to fix this. Under current law, if Bob dies before Sue, his unused exemption can be transfered to Sue. But if Sue remarries and outlives her new husband, she would lose Bob’s unused exemption. Also, by leaving everything to Sue, Bob has no control over how his share of the assets are managed or distributed. Plus, any growth on the assets will be included in Sue’s estate and taxed when she dies.

If Bob and Sue plan ahead, they can use both exemptions and solve these problems. A tax-planning provision in their living trust splits their $10 million estate into two trusts of $5 million each. When Bob dies, his trust uses his $5 million exemption. When Sue dies, her trust uses her $5 million exemption. This reduces their taxable estate to $0, so the full $10 million can go to their loved ones.

In addition, Bob can keep control over how his share of the estate is managed and distributed; the assets are valued and taxed only at his death, so no growth is included in Sue’s estate; yet, the assets in Bob’s trust can be available for anything Sue needs. Married couples with estates of all sizes find these benefits appealing. (This planning can also be done in a will, but you would not avoid probate or enjoy the other benefits of a living trust.)