Mutual wills: equity can enforce agreed irrevocable testamentary interests despite a surviving party's later revocation. A will takes effect after death and creates no executor rights in the testator's lifetime. Joint wills are revocable by either testator or the survivor unless an enforceable agreement not to revoke is proved, and identical terms alone do not establish such a contract. Mutual wills arise from an agreement to create irrevocable testamentary interests; although formally revocable, equity may enforce those agreed interests against a surviving party who breaches the agreement, with personal representatives holding the estate on trust to effect the agreed dispositions.