There is no legal definition of ‘mirror Will’, and the term sometimes means slightly different things to different people. However, the various uses have certain things in common:
two people who wish to make essentially the same provision for each other, or for each other together with other beneficiaries;
the couple usually make their Wills at the same time (though this isn't a requirement); and
the Wills are separate documents.
In other words, mirror Wills are individual Wills made by two people (usually married, in a civil partnership or cohabiting) that are intended to match or be practically identical.
The common situation is that mirror Wills are made by a couple who are married or in a civil partnership. Typically, mirror Wills will leave everything to the other person, and if they have already died, everything to their children.
But there is no legal reason why more complicated provision cannot be made in mirror Wills. Neither is there any legal reason why any two people (not just married couples or civil partners) cannot make Wills which match each other in this way.
For example, if you live together as a couple in a permanent relationship, but are not married or civil partners, you might still have virtually identical wishes as to what you would like to happen to your property when you die. For such a couple, mirror Wills could be doubly important:
to give effect to your joint wishes; and
to avoid the effects of intestacy – because the rules of intestacy would mean that without Wills your estates might be distributed very differently.
Effect of mirror Wills
It does not matter whether a Will is described as a mirror Will or not, as it has no effect in law on the validity of the Will or on how the clauses in the Will are to be put into effect. This is because each person can only make provision in relation to their own estate. Even if you intend to make the same provision as your partner, each of you is making an independent choice, so each Will is separate. Remember, 'mirror Wills' is simply a way of describing separate Wills made with the intention that their provisions reflect each other (they don't have to be identical).
And there is nothing to stop either of you from making an entirely new Will, whether or not your partner is still alive. The only exception is in the case of mutual Wills (see below).
So making mirror Wills is often a matter of practical convenience, and couples use them for clarity and simplicity, and to make the drafting of them match for consistency. However, they are not always suitable.
Joint Wills
People often speak about making ‘joint Wills’ when they want to make matching provision to each other - such as a husband leaving everything to his wife, and vice versa. It is perfectly acceptable to make Wills which make that provision, but in law a Will can relate to only one person, so that matching provision has to be made in two Wills. Even if two people were to try to make a joint Will in a single document, the law would treat it as two Wills. Most people use the term ‘joint Wills’ to refer to some form of mirror Will.
Complementary Wills
You may sometimes see the term ‘complementary Wills’, or occasionally ‘matching Wills’, and these can both describe the same situation as mirror Wills.
Mutual Wills
But however it is described, ‘mirror Wills’ should not be confused with ‘mutual Wills’. Mutual Wills are rarer and more complex, and involve binding provisions that prevent one of the testators from subsequently changing their Will.