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 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 mnemonic PITT is used for the four unities here: Possession, Interest, Time, & Title.
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. Realism and Formalism in the Severance of Joint Tenancies This is because, for the purpose of the property interest, joint owners are regarded as a single legal entity.