RCW 9.41.250
Dangerous weapons — Penalty.
Every person who:
(1) Manufactures, sells, or disposes of or possesses any instrument or weapon of the kind usually known as slung shot, sand club, or metal knuckles, or spring blade knife, or any knife the blade of which is automatically released by a spring mechanism or other mechanical device, or any knife having a blade which opens, or falls, or is ejected into position by the force of gravity, or by an outward, downward, or centrifugal thrust or movement;
RCW 9.41.270
Weapons apparently capable of producing bodily harm — Unlawful carrying or handling — Penalty — Exceptions.
(1) It shall be unlawful for any person to carry, exhibit, display, or draw any firearm, dagger, sword, knife or other cutting or stabbing instrument, club, or any other weapon apparently capable of producing bodily harm, in a manner, under circumstances, and at a time and place that either manifests an intent to intimidate another or that warrants alarm for the safety of other persons.
(here's the exception)
(c) Any person acting for the purpose of protecting himself or herself against the use of presently threatened unlawful force by another, or for the purpose of protecting another against the use of such unlawful force by a third person;
SMC 12A.14.010 Definitions.
The following definitions apply in this chapter:
A. "Dangerous knife" means any fixed-blade knife and any other knife
having a blade more than three and one-half inches (3 1/2") in length.