A more thorough explanation:

Definition: The four qualities required to create a joint tenancy at common law are interest, possession, time, and title. These are known as the four unities.

Unity of interest: All joint tenants' interests must be identical in nature, extent, and duration.

Unity of possession: Each joint tenant must be entitled to possession of the whole property.

Unity of time: All joint tenants' interests must vest at the same time.

Unity of title: All joint tenants must acquire their interests under the same instrument.

For example, if three people buy a house together and each person has an equal share in the property, they have unity of interest. If each person has the right to use and occupy the entire property, they have unity of possession. If they all bought the property at the same time, they have unity of time. If they all signed the same purchase agreement, they have unity of title.

These four unities are important because they establish the legal relationship between joint tenants and determine how the property will be distributed if one of the tenants dies.