A Mutual Will Binds the Survivor Who Accepts It

A will is revocable by definition - a testator can tear one up and write another at any point until death. Mutual wills are the exception, and couples who make them frequently don’t realise they have created one.

The mechanism has two elements. Two testators - typically spouses - execute wills on agreed terms, usually leaving assets to each other and then onward to the same eventual beneficiaries. Alongside that, there is an agreement between them not to revoke those terms. When the first dies and the survivor takes a benefit under the arrangement, the survivor becomes bound by it. Courts give effect to this by imposing a constructive trust over the property, so a later will attempting different terms doesn’t defeat the original agreement.

The Agreement Can Arise by Implication, Not Just by Clause

The part that catches people is what counts as the agreement. It doesn’t have to be a separate contract, or a clause expressly saying “neither of us may revoke.” It can arise by necessary implication from the structure of a single joint document - from terms that only make sense if both parties intended them to be irrevocable. A couple executing one will together, understanding it as a tidy way to record a shared intention, may have created a binding obligation on whichever of them lives longer.

A Joint Will and a Mutual Will Look Nearly Identical

The distinction that matters is between a joint will that is simply two wills in one document - each freely revocable, no agreement between them - and a mutual will carrying the non-revocation element. They can look nearly identical on the page. What separates them is whether an agreement not to revoke can be established, and that question gets litigated after the person who could have clarified it has died.

For a couple who genuinely want the terms locked - protecting children from an earlier marriage against the survivor redirecting assets later - that binding quality is the whole point, and worth stating explicitly rather than leaving to implication. For a couple who simply want convenience and expect to retain full freedom afterwards, separate wills avoid creating an obligation neither of them intended.

Say which one you mean, in the document. If the point is to lock the terms - protecting children from an earlier marriage - state the non-revocation expressly rather than leaving it to implication. If the point is convenience with freedom retained, use separate wills and create no obligation at all.