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 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.
In common law, the "time" requirement could be satisfied only by using a "straw man" 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 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.

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

References

Property law