MUTUAL WILLS: arise where 2 people make wills in similar terms, agreeing whichever of them survives will irrevocably leave their estate in a particular way. If X dies first as will is revocable Y could revoke the mutual will make a new one on different terms. However, given X fulfilled her side of the bargain, it is unconscionable for Y to go back on promise. So, equiy will step in form of doctrine of mutual wills and impose a constructive trust over the husband's estate in favour of mutual will. Making identical wills is insufficient to make them mutual wills > for the doctrine to apply there must have been an agreement, Wills must have been made as a clear agreement between testators as tot he disp[osal of their estates and part of that agreement is that aurvuvor will not revoke their will. constructive trust arises when first testator dies without hvaing revoked their will. At that point, the first testator has carried out their side of the bargain, so it would be wrong to allow survivor to renege on their side.