When two or more persons are equally owners of some property.
The unique aspect of 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 valid joint tenancy is said to require the "four unities": unity of interest (each joint tenant must have an equal interest including equality of duration and extent), unity of title (the interests must arise from the same document), unity of possession (each joint tenant must have an equal right to occupy the entire property) and unity of time: the interests of the joint tenants must arise at the same time.
If the instrument which purports to create a joint tenancy is ambiguous, that could lead to the Court reading into it a tenancy in common. As was stated by Justice Joyce in Re Wooley:
"(I)n a case of ambiguity, the Court very properly leans to a construction which creates a tenancy in common in preference to a joint tenancy.
"... anything which in the slightest degree indicates an intention to divide the property must be held to abrogate the idea of a joint tenancy."
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