Unity of time 

Interest must be acquired by both tenants at the same time.

In common law, the "time" requirement could be satisfied only by using a "straw man" to create a joint tenancy. The party creating the joint tenancy would have to convey title to a straw man, who would then transfer title to the two parties as joint tenants.

Unity of title 

The interests held by the co-owners must arise out of the same instrument.^[1]

Unity of interest 

Both tenants must have the same interest in the property.

This means that the joint tenants must have the same type of interest, and the interest must run for the same duration. For example, if X and Y create a joint tenancy, both X and Y's interests must be in fee simple absolute. If, for example, X has a fee simple absolute and Y has a life estate, there is no unity of interest.

Unity of possession 

Both tenants must have the right to possess the whole property.

If any of the four unities is broken and it is not a joint tenancy, the ownership reverts to a tenancy in common.

The unique aspect of a joint tenancy is that as the joint tenancy owners die, their shares accrue to the surviving owner(s) so that, eventually, the entire share is held by one person.