A joint Will is a single Will made by two people which deals with the estates of both of them. Usually it will leave everything to the other on the first death and then to agreed beneficiaries on the death of the survivor.
Mutual or Mirror wills are where the parties have entered into a legally enforceable agreement to make a Will in agreed terms. If on the first death the survivor finds that the deceased had revoked or altered their Will they could make a claim on the estate for breach of contract.

Partners and married couples who have been together for a long time and share the same thoughts and ideas sometimes consider making joint or mutual Wills. There are advantages but many disadvantages of both Joint and mutual Wills. So should you and your partner consider making one. They can be convenient and an ideal way to ensure that when you die your partner will be taken care of during their lifetime and on their death you will not have to worry about your estate not passing to your children. If your partner should remarry they would not be able to change their Will and disinherit your children.

This book provides a guide to the grounds on which a will may be contested, with examples of real cases.

A guide to the grounds on which a will may be contested.

However, the inability to change the provisions of a Will can also be a disadvantage. Circumstances change. The ability of the survivor to deal with changing circumstances will be severely restricted both with regard to their property and the inherited property. An adult child could not be given an inheritance early to buy a house or start a business. Grandchildren could not be helped with their education. A home could not be sold to buy something smaller. Tax saving gifts could not be made.

For these reasons the balance of opinion is against Joint or Mutual Wills and they are rarely made.