Four unities

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.
The four unities.
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.