The traditional law relating to severance has, at least since Blackstone, commonly been seen as deducible from the four unities - time, possession, title and interest - which are hallmark inhabitants of the institution of joint tenancy. Where all four unities are present in a multiple holding of land, there is joint tenancy. If one or other of them be absent, there ordinarily is not. The "captivating appearance of symmetry and exactness" (Challis's Law of Real Property, 3rd ed. (1911), p. 367) of this traditional ritual cloaks some obscurity of precise meaning, some overlapping between the unities and some conceptual difficulties about the essential character of joint tenancy. In addition, the analysis requires qualification in that there are cases where the absence of unity of time will be excused (see, e.g., Kenworthy v. Ward [23] ), where one or other of the joint tenants may enjoy contractual rights of separate possession of the whole or part of the property without severance of the underlying joint tenancy and where surrender of possession by one joint tenant to a third party under a lease of his interest for a term will lead to suspension rather than severance of the joint tenancy (see, e.g., Wright v. Gibbons [24] ). Nonetheless, at law, where time, possession, title and interest can be seen as of the stuff of property, the question whether there has been a severance of a joint tenancy by some dealing with, or operation of law upon, one joint tenant's share can, subject to such qualification, be equated with the question whether one or more of the unities of possession, title and interest has been destroyed: in that equation, the unity of time can be disregarded as a practical matter since, if unity of title, interest and possession remain undisturbed, the original unity of time will persist. In equity, where good conscience and actual or presumed intention may prevail over common law rights and interests, and tenancy in common is seen as a preferred instrument for the reason that it avoids the gamble of the tontine, particular care must be taken to ensure that the equation of those questions does not divert attention from principle and from the two aspects of joint tenancy which are most likely to attract the operation of overriding equitable doctrine, namely, (i) the equality of the interests of joint tenants, regardless of intention or contribution, in the undivided rights constituting ownership of the relevant property, and (ii) the right of accretion by survivorship until there is a sole owner of the whole. Where legal joint tenancy persists, severance in equity must involve the creation of some distinct beneficial interests, that is to say, the creation of a trust for the joint tenants themselves as tenants in common in equal shares or for different beneficiaries or beneficial shares (see, e.g., Gissing v. Gissing [25] ; Allen v. Snyder [26] ). In such a case, the imposition of a trust for tenants in common is likely to be the result of the direct operation of applicable equitable doctrine and the question whether the destruction in equity of one or more of the four unities has brought about a severance in equity of the joint tenancy may be an unhelpful and even confusing starting point. Accordingly, it seems to me that the preferable starting point in the present case is a more general inquiry about whether the effect of the operation of any applicable doctrine of equity was, as between Mrs. Patton and Mr. Corin, to give rise to a trust of any interest in the subject land. If it was, the question will then arise whether the effect of that trust or beneficial interest was to create a tenancy in common of the subject land in equity which bound Mr. Patton and effectively precluded him from enjoying the benefit of the right of survivorship which he enjoyed at law.