The four unities is a concept in the common law

Common law

Common law is law developed by judges through decisions of courts and similar tribunals rather than through legislative statutes or executive branch action...

 of real property

Real property

In English Common Law, real property, real estate, realty, or immovable property is any subset of land that has been legally defined and the improvements to it made by human efforts: any buildings, machinery, wells, dams, ponds, mines, canals, roads, various property rights, and so forth...

 describing conditions that must exist in order for certain kinds of property interests to be created. Specifically, 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, they must have the following unities:

The four unities

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

Strawperson

A strawperson or straw man is a figure not intended to have a genuine beneficial interest in a property, to whom such property is nevertheless conveyed in order to facilitate a more complicated transaction at law....

" 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.

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 tenancy by the entirety, the ownership reverts to a tenancy in common.

A fifth unity

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.