Washington Revised Code Title 46. Motor Vehicles § 46.87.070. Reciprocity for trailers, semitrailers, pole trailers

Trailers, semitrailers, and pole trailers properly based in jurisdictions other than Washington and displaying currently registered license plates issued by the jurisdictions are granted vehicle registration reciprocity in this state. Unless registered under the provisions of the IRP as a pool fleet, such trailers, semitrailers, and pole trailers must be operated in combination with an apportioned power unit to qualify for reciprocity. If pole trailers are not required to be licensed separately by a member jurisdiction, they may be operated in this state without displaying a base license plate.