What is the defining characteristic of joint tenancy that distinguishes it from tenancy in common?

The hallmark of joint tenancy is the right of survivorship: on the death of a joint tenant, that share passes automatically to the surviving joint tenants and not through the deceased's will or to their heirs. Joint tenancy traditionally requires the four unities — time, title, interest, and possession — meaning the owners take equal interests through the same deed at the same time. Tenancy in common, by contrast, allows unequal shares, has no survivorship, and a co-tenant's interest passes to their heirs or devisees. Understanding survivorship is essential because it determines how title moves on death without probate.