At first sight, the answer appears to be 'not a great deal'. However, the key point of difference in mutual wills is that they are a legally binding and enforceable contractual agreement by the testators not to alter their respective wills in any way at any time and especially not after the death of the first testator. On the other hand, each party to a mirror will fully accepts the right of the survivor to change his or her will. There is no real legal sanction possible with a mirror will to prevent the testators changing their respective wills - they simply have to trust that it doesn't happen. Of course, even with a standard will, there are some consequences if you don't live up to commitments made, especially if you'd given a verbal or written promise to someone assuring them that they'll inherit property or land. If they acted and relied on that promise, they can use the challenge ofproprietary estoppel to have that promise honoured if it's revoked or not mentioned in your will.