Wash. Rev. Code § 64.04.010
Every conveyance of real estate, or any interest therein, and every contract creating or evidencing any encumbrance upon real estate, shall be by deed: PROVIDED, That when real estate, or any interest therein, is held in trust, the terms and conditions of which trust are of record, and the instrument creating such trust authorizes the issuance of certificates or written evidence of any interest in said real estate under said trust, and authorizes the transfer of such certificates or evidence of interest by assignment by the holder thereof by a simple writing or by endorsement on the back of such certificate or evidence of interest or delivery thereof to the vendee, such transfer shall be valid, and all such assignments or transfers hereby authorized and heretofore made in accordance with the provisions of this section are hereby declared to be legal and valid.
RCW 64.04.010