# A-B Trust: Definition, How It Works, and Tax Benefits

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## What Is an A-B Trust?

An A-B trust is a joint trust created by a married couple in order to minimize estate taxes. It is formed with each spouse placing [assets](https://www.investopedia.com/terms/a/asset.asp) in the [trust](https://www.investopedia.com/terms/t/trust.asp) and naming the final [beneficiary](https://www.investopedia.com/terms/b/beneficiary.asp)—that is, anyone except the other spouse. The trust gets its name from the fact that it splits into two separate entities when one spouse dies. Trust A is the survivor's trust and trust B is the decedent's trust. A-B trusts have become less popular because the estate tax exemption covers most estates.  

### Key Takeaways

*   An A-B trust is a joint trust created by a married couple.
*   Upon one spouse's death, the trust splits into a survivor portion (the A trust) and a bypass portion (the decedent's trust, or B trust).
*   Via the split, the A-B trust effectively minimizes estate taxes and defers them until after the death of the surviving spouse.
*   The surviving spouse has limited control over the decedent's trust but the terms of the decedent's trust can be set to allow the surviving spouse to access the assets, and even draw income from them.
*   A-B trusts are no longer widely used as the estate tax exemption, which is now indexed to inflation, is sufficient for most estates.

## Understanding an A-B Trust

[Estate taxes](https://www.investopedia.com/terms/e/estatetax.asp) can take a sizable portion of a deceased person's assets. For example, consider a married couple who has an estate worth $20 million by the time one of the spouses dies. The surviving spouse is left with the whole $20 million, which is not taxed due to the [unlimited marital deduction](https://www.investopedia.com/terms/u/unlimited-marital-deduction.asp) for assets flowing from a deceased spouse to a surviving spouse.

But then, the other spouse dies, leaving the money to their children. The taxable portion of the estate (the amount that exceeds the 2025 exemption threshold of $13.99 million will be $6.01 million). This means that $6.01 million will be taxed at 40% ($3.61 million), leaving $17.6 million ($13.99 million + $3.61 million) for the beneficiaries.  

To circumvent an estate from being subject to such steep taxes, many married couples set up a trust under their [last will and testaments](https://www.investopedia.com/terms/l/last-will-and-testament.asp) called an A-B trust. In the example above, if the couple instead had an A-B trust, the death of the first spouse would not trigger any estate taxes as a result of the lifetime exclusion; however, a sum of money equal to the current exemption amount ($13.99 million) will be transferred into an [irrevocable trust](https://www.investopedia.com/terms/i/irrevocabletrust.asp) called the bypass trust or B trust. This trust is also known as the decedent’s trust. The remaining amount, $6.01 million, will be transferred to a survivor’s trust, or A trust, which the [surviving spouse will have complete control](https://www.investopedia.com/terms/m/marital-trust.asp) over. The estate tax on the A trust is deferred until after the death of the surviving spouse.

It's important to note that if the surviving spouse elected portability when the first spouse died, the portion of the first spouse's exemption would pass to the surviving spouse, in this case, $13.99 million, which means that the entire $20 million would pass tax-free to the beneficiaries, as the combined exemption of both spouses would be $27.98 million. This removes the need for an A-B trust.

## Benefits of an A-B Trust

The A trust contains the surviving spouse’s property interests, but they have limited control over the assets in the deceased spouse's trust. However, this limited control over the B trust will still enable the surviving spouse to live in the couple's house and draw income from the trust, provided these terms are stipulated in the trust.

While the surviving spouse can access the bypass trust, if necessary, the assets in this trust will bypass their taxable estate after they die. After the surviving spouse dies, only the assets in the A trust are subject to estate taxes. If the [estate tax exemption](https://www.investopedia.com/estate-tax-exemption-2021-definition-5114715) for this spouse is also $13.99 million for 2025 and the value of assets in the survivor’s trust is still valued at $6.01 million, none of it will be subject to estate tax.

The federal tax exemption is transferable between married couples through a designation referred to as the portability of the estate tax exemption. If one spouse dies, the unused portion of their estate tax exemption can be transferred and added to the estate tax exemption of the surviving spouse. Upon the death of the surviving spouse, the property in the decedent's trust passes tax-free to the beneficiaries named in this trust.

This is because the B trust uses up the estate tax exemption of the spouse who died first, so any funds left in the decedent’s trust will be passed tax-free.

> **Important**
>
> The decedent’s trust is not considered part of the surviving spouse's estate for purposes of the estate tax, so double taxation is avoided.

## Who Uses an A-B Trust?

While A-B trusts are a great way to minimize estate taxes, they are not used much today. They were popular when the estate tax—which hadn't been adjusted for years—could be triggered on estates as small as $1 million or $2 million. Nowadays, each individual has a combined lifetime [federal gift tax](https://www.investopedia.com/terms/g/gifttax.asp) and estate tax exemption of $13.99 million ($27.98 million for married couples) in 2025.

So only people with estates valued over $13.99 million ($27.98 million for married couples) will opt for an A-B trust in 2025. With the portability provision, a surviving spouse can include the tax exemption of their late spouse, which can be transferred tax-free to beneficiaries if filed within the 9-month period.

## What Are the Benefits of an A-B Trust?

The benefits of an A-B trust include [death tax](https://www.investopedia.com/terms/d/death-taxes.asp) exemptions, built-in trust protection, and the portability of exemption. Disadvantages include maintenance costs, complex structure, and the possibility of large [capital gains](https://www.investopedia.com/terms/c/capitalgain.asp) taxes after both spouses die.

## Why Is an A-B Trust Obsolete?

A-B trusts aren't completely obsolete, but they aren't as common as they once were. This is due to changes in estate tax law—that is, couples now have high estate tax exemptions. (In 2025, it's $13.99 million for an individual, or $27.98 million for a married couple who uses portability of exemption.)

## What Is Another Name for an A-B Trust?

A-B trusts are also known as bypass trusts or credit shelter trusts. The goal of these trusts is to reduce married couples' estate taxes.

## The Bottom Line

A-B trusts are beneficial in [estate planning](https://www.investopedia.com/terms/e/estateplanning.asp) to reduce taxes. However, they are less popular now that there are higher estate tax exemptions. (In 2025, it's $13.99 million for an individual, and $27.98 million for a married couple who uses portability of exemption.) For those with [estates](https://www.investopedia.com/terms/e/estate.asp) that exceed these limits, though, it's likely worth it to work with an advisor who can help you find ways to minimize taxes when passing on your assets to your [heirs](https://www.investopedia.com/terms/h/heir.asp).

> **Article Sources**
>
> 1.  Internal Revenue Service. "[Estate Tax](https://www.irs.gov/businesses/small-businesses-self-employed/estate-tax)."
>     
> 2.  Internal Revenue Service. "[Frequently Asked Questions on Estate Taxes](https://www.irs.gov/businesses/small-businesses-self-employed/frequently-asked-questions-on-estate-taxes)."