A more thorough explanation:

Mutual wills are wills made by two or more people, usually spouses or committed couples. These wills are reciprocal, identical, or substantially similar. The purpose of mutual wills is to grant the survivor of the two the property contained in the wills, and then to specified individuals after the death of the survivor.

For example, John and Jane are a married couple. They create mutual wills that state that if one of them dies, the other will inherit all of their property. After the death of the survivor, their property will be distributed to their children.

Mutual wills can contain clauses that they are not revocable without the consent of both parties. Without such a clause, mutual wills do not automatically carry the presumption of a contract.

For example, if John and Jane's mutual wills do not contain a clause that they are not revocable without the consent of both parties, John can change his will after Jane's death.

If a mutual will has been properly created, a surviving spouse cannot later change the terms of the will or make an inter vivos transfer of funds. Prior to the death of the first spouse, however, either spouse can make changes to their will as long as they first notify the other spouse that they have done so.

For example, if John and Jane's mutual wills contain a clause that they are not revocable without the consent of both parties, John cannot change his will after Jane's death. However, before Jane's death, either John or Jane can make changes to their will as long as they first notify the other spouse that they have done so.