An appreciation of the typical mutual wills arrangement often suffices for a judge who has to apply the mutual wills doctrine, as well as for commentators who write about particular aspects of the doctrine. As a result, there has been little impetus to pinpoint the precise factual ambit of the doctrine's operation, that is, to identify the specific real-world events which trigger the mutual wills doctrine. However, the need to do so has now come into sharp focus, in view of the Law Commission's plan to review the law concerning wills, including mutual wills. Certainly, a successful review hinges on an accurate definition of what a "mutual wills arrangement" entails. However, the typical arrangement provides little assistance to this end. Many factual variations have not prevented the mutual wills doctrine from operating, for instance, where T1 leaves property to T2 inter vivos, where T1 does not leave property to T2 at all, and where the parties want their agreement to be binding even before either of them have died. A further difficulty arises in relation to the terminology used. Despite the "mutual wills" label, a constructive trust is neither triggered merely by the mutuality of the terms of the testators' wills nor the mutuality of the timing at which their wills are executed. Indeed, it appears not even to be necessary for the arrangement to involve the making of wills at all. This paper considers the many possible factual variations and accounts for them in order to state precisely the ambit of the mutual wills doctrine
