Four unities

The four unities is a concept in the common law of real property that describes conditions that must exist in order to create certain kinds of property interests. Specifically, these four unities must be met for two or more people to own property as Joint tenancy#Joint [tenancy with right of survivorship|joint tenants with legal right of survivorship], or for a married couple to own property as tenants by the entirety. Some jurisdictions may require additional unities.

The four unities

The mnemonic PITT is used for the four unities here: Possession, Interest, Time, & Title.
; Unity of time : Interest must be acquired by both tenants at the same time.
; Unity of title : The interests held by the co-owners must arise out of the same instrument.
; Unity of interest : Both tenants must have the same interest in the property.
; 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 so that, eventually, the entire share is held by one person.

A fifth unity

; Unity of marriage : For a tenancy by the entirety this fifth unity must be present. Marriage combined with the preceding four unities creates a tenancy by the entirety. A tenancy by the entirety gives rise to certain legal rights, such as rights of survivors, when one spouse is deceased that interest automatically passes to the surviving spouse. Additionally, in many States, the creditor of only one of the spouses cannot take the property held as tenants by the entirety; both spouses must be indebted to the creditor.

A sixth unity

; Unity of unison : For the parties to unite, they must be in unison. This has been criticised by the Law Commission in their 283rd report, entitled 'Unity in Leaseholds'.