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 on his or her death (unless he or she revokes (cancels) the will during his or her 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 his or her ability to dispose of his or her property by the agreement he or she 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:

1. The agreement must be made in a particular form.

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

3. The agreement must be intended to be irrevocable.

4. 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. It is also noted 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.

A will contract is a term used in the law of wills describing a contract to exchange a current performance for a future bequest. In such an agreement, one party (the promise) will provide some performance in exchange for a promise by the other party (the testator, because they must draft a will) to make a specific bequest to the promise party in the testator's will. Most jurisdictions recognize such contracts as valid, although a few hold them as void against public policy. Some jurisdictions will not recognize an oral contract for such a purpose, requiring instead that the contract be executed in writing and signed by both parties. Some jurisdictions require full compliance with the Statute of Wills to be effective, i.e., in writing and signed in the presence of two witnesses.

A codicil is a testamentary document similar but not necessarily identical to a will. In some jurisdictions, it may serve to amend, rather than replace, a previously executed will. In others, it may serve as an alternative to a will. In still others, there is no recognized distinction between a codicil and a will.