Joint wills and mutual wills are closely related terms used in the law of wills to describe two types of testamentary writing that may be executed by a married couple to ensure that their property is disposed of identically. Neither should be confused with mirror wills which means two separate, identical wills, which may or may not also be mutual wills.

Joint wills.

A joint will is a single document executed by more than one person (typically between spouses), making which has effect in relation to each signatory's property upon death (unless the will is revoked (cancels) the will during the signatory's lifetime). Although a single document, the joint will is a separate distribution of property by each executor (signatory) and will be treated as such on admission to probate. Mutual wills are any two (or more) wills which are mutually binding, such that following the first death the survivor is constrained in the ability to dispose of the property by the agreement the survivor made with the deceased. Historically such wills had an important role in ensuring property passed to children of a marriage rather than a spouse of a widow or widower on a remarriage.

The recognition of these forms varies widely from one jurisdiction to the next. Some permit both, some will not recognize joint wills, and many have established a presumption that one or both of these forms creates a will contract.

A joint will differs substantively from a mutual will in that the former is not intended to be irrevocable or to express a mutual intention; it is merely an administrative convenience. A will may be both joint (on one document) and mutual (see below).

Mutual wills.

Mutual wills have four basic requirements and a strict standard for enforceability:

The agreement must be made in a particular form.

The agreement must be contractual in effect. (Contrast Goodchild v Goodchild and Lewis v Cotton).

The agreement must be intended to be irrevocable.

The surviving party must have intended the will to reflect the agreement.

Mutual wills are rare, and often another form of constructive trust is imposed (Healey v Browne). It is also noted (Carnwath J in Re Goodchild) that a mutual will is a technical legal device requiring an intention to form a binding agreement and that this often differs from the "loose moral obligation" presupposed as binding by the layman.