The four unities is a concept in the common law of real property describing conditions that must exist in order for certain kinds of property interests to be created. Specifically, these four unities must be met in order for two or more people to own property as joint tenants with right of survivorship, or for a married couple to own property as tenants by the entirety. Some jurisdictions may require additional unities.
Unity of time Four unities Wikipedia
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(s) so that, eventually, the entire share is held by one person.Unity of marriage
In order for there to be 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.
Unity of unison
In order for the parties to be united, they must be in unison. This has been criticised by the Law Commission in their 283rd report, entitled 'Unity in Leaseholds'.