2. In a joint estate there must exist four unities; that of interest, for a joint-tenant cannot be entitled to one period of duration or quantity of interest in lands, and the other to a different; one cannot be tenant for life, and the other for years: that of title, and therefore their estate must be created by one and, the same act; that of time, for their estates must be vested at one and the same period, as well as by one and the same title; and lastly, the unity of possession: hence joint-tenants are seised per my et per tout, or by the half or moiety and by all: that is, each of them has an entire possession, as well of every parcel as of the whole. 2 Bl. Com. 179-182; Co. Litt. 188.
3. Coparceners must have the unities of interest, title, and possession.
4. In tenancies in common, the unity of possession is alone required. 2 Bl. Com. 192; 2 Bouv. Inst. n. 1861-83. Vide Estate in Common; Estate in Joint-tenancy; Joint-tenants; Tenant in Common; Tenants, Joint.