A-B Trust: Definition, How It Works, and Tax Benefits
By Imperialpedia Staff
What Is an A-B Trust?
An A-B trust is an estate-planning structure created by a married couple that splits into two separate trusts when the first spouse dies. Trust A (sometimes called the survivor's trust or marital trust) holds the surviving spouse's share and remains under their control. Trust B (the decedent's trust, sometimes called a bypass or credit-shelter trust) holds the deceased spouse's share, uses their available estate-tax exemption, and passes to the couple's chosen final beneficiaries — typically their children — rather than to the surviving spouse outright.
Why Couples Used to Set These Up
The core purpose of an A-B trust is to make sure both spouses' individual federal estate-tax exemptions get used, rather than only the surviving spouse's. Without this structure, if the first spouse's assets pass directly to the survivor, their personal exemption can effectively go unused, potentially leaving a larger combined estate exposed to estate tax when the second spouse later dies.
IMPORTANT
Because assets held in Trust B are not counted as part of the surviving spouse's estate, they avoid being taxed a second time when that spouse later passes away — one of the main tax benefits of the structure.
Why A-B Trusts Have Become Less Common
Since 2011, U.S. tax law has allowed a surviving spouse to claim any unused portion of a deceased spouse's federal estate-tax exemption directly, through an election called "portability," without needing a trust to preserve it. Combined with a federal exemption threshold that now covers the vast majority of estates, this has significantly reduced how often an A-B trust is strictly necessary purely for federal estate-tax purposes. A-B trusts can still serve other goals, however — such as controlling how assets are ultimately distributed, protecting assets for children from a prior marriage, or addressing state-level estate taxes with lower exemption thresholds than the federal one.
A Simplified Example
Suppose a married couple has a combined estate and the first spouse dies. Their share of the assets, up to their available exemption amount, funds Trust B and bypasses the surviving spouse's taxable estate entirely. The surviving spouse can typically still receive income from Trust B, and often has some access to principal under specific terms, but the assets themselves are earmarked for the couple's chosen final beneficiaries and are not taxed again in the survivor's estate.
Considerations Before Setting One Up
- Portability may already achieve the core federal tax benefit without a trust, depending on the estate's size.
- State estate taxes often have much lower exemption thresholds than the federal exemption, which can still make this structure useful.
- Trust B's terms limit the surviving spouse's flexibility with those assets compared to owning them outright.
- This is a complex area of estate law — a qualified estate-planning attorney should structure any trust based on the couple's specific situation.
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