California Code, Civil Code - CIV § 683

(a) A joint interest is one owned by two or more persons in equal shares, by a title created by a single will or transfer, when expressly declared in the will or transfer to be a joint tenancy, or by transfer from a sole owner to himself or herself and others, or from tenants in common or joint tenants to themselves or some of them, or to themselves or any of them and others, or from spouses, when holding title as community property or otherwise to themselves or to themselves and others or to one of them and to another or others, when expressly declared in the transfer to be a joint tenancy, or when granted or devised to executors or trustees as joint tenants. A joint tenancy in personal property may be created by a written transfer, instrument, or agreement.

(b) Provisions of this section do not apply to a joint account in a financial institution if Part 2 (commencing with Section 5100) of Division 5 of the Probate Code applies to such account.