A more thorough explanation:

Mirror wills are wills created by two people that have almost the same provisions. They are usually made by married couples. Each spouse's will "mirrors" the other spouse's will. For example, a married couple's mirror wills may each say that the surviving spouse gets the deceased spouse's entire estate. Mirror wills are not necessarily binding, and each spouse can change their will without asking the other spouse. They are also used by siblings, unmarried partners, or business partners.

• John and Jane are a married couple. They create mirror wills that say that if one of them dies, the other spouse gets everything. If they both die, their children get everything.
• Two sisters, Sarah and Emily, create mirror wills that say that if one of them dies, the other sister gets everything. If they both die, their parents get everything.
• Two business partners, Tom and Jerry, create mirror wills that say that if one of them dies, the other partner gets everything. If they both die, their company gets everything.

These examples illustrate how mirror wills work. In each case, the wills are almost identical, and each person's will mirrors the other person's will. If one person dies, the other person gets everything. If both people die, their assets go to someone else.