Wash. Rev. Code § 11.02.110
Whenever shares or other securities issued by domestic or foreign corporations are or have been issued or transferred to two or more persons in joint tenancy form on the books or records of the corporation, it is presumed in favor of the corporation, its registrar and its transfer agent that the shares or other securities are owned by such persons in joint tenancy and not otherwise. A domestic or foreign corporation or its registrar or transfer agent is not liable for transferring or causing to be transferred on the books of the corporation to or pursuant to the direction of the surviving joint tenant or tenants any share or shares or other securities theretofore issued by the corporation to two or more persons in joint tenancy form on the books or records of the corporation, unless the transfer was made with actual knowledge by the corporation or by its registrar or transfer agent of the existence of any understanding, agreement, condition, or evidence that the shares or securities were held other than in joint tenancy, or of the invalidity of the joint tenancy or a breach of trust by the joint tenants.
RCW 11.02.110