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The answer is - it depends.
It is illegal to own a switchblade or other spring blade knife in the state of Washington. So if the pocket knife is a switchblade or a spring blade, it is illegal.
In addition, it is illegal to carry any "dangerous weapon" (which can be defined very, very broadly under State v. Bonebright as anything that can inflict great bodily harm or cause death) which CAN include a pocket knife regardless of blade length; as well as a knife, or other cutting or stabbing instrument in a manner or under circumstances that would cause alarm or show an intent to intimidate another.
As such, while there is no specific law that states "no pocket knives," the authorities can prosecute a case against someone carrying a pocket knife via the above laws, regardless. However, the authorities are only likely to do this if they come across the knife during another illegal act by the person, or, the person is carrying the knife in a way that may cause others alarm.
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