One of the main reasons why real property held in joint tenancy is not preferable is due to its inflexible nature. If one joint tenant dies, the surviving joint tenant(s) automatically receive the interest of the deceased joint tenant. This is true even if the joint tenant wrote a will and devised their interest in the property to somebody other than the surviving joint tenant.
In order to avoid the automatic transfer upon a joint tenant's death, severance has to occur. Civil Code § 683.2 provides various methods in which a joint tenancy can be severed:
(2) Execution of a written instrument that evidences the intent to sever the joint tenancy, including a deed that names the joint tenant as transferee, or of a written declaration that, as to the interest of the joint tenant, the joint tenancy is severed."