Get a copy of your State and County's vehicle equipment requirement laws.

I've often wondered the same thing myself, Sam.
Where might I find the license plate law (WA State)? RCW 46.16.010 is the closest I have been able to find, and it merely states "(1) It is unlawful for a person to operate any vehicle over and along a public highway of this state without first having obtained and having in full force and effect a current and proper vehicle license and display vehicle license number plates therefor as by this chapter provided." But it remains a mystery to me where this chapter doth provide the aforementioned info...

The vehicle license number plates shall be attached conspicuously at the front and rear of each vehicle for which the same are issued and in such a manner that they can be plainly seen and read at all times. However, if only one license number plate is legally issued for any vehicle such plate shall be conspicuously attached to the rear of such vehicle. Each vehicle license number plate shall be placed or hung in a horizontal position at a distance of not more than four feet from the ground and shall be kept clean so as to be plainly seen and read at all times. In cases where the body construction of the vehicle is such that compliance with this section is impossible, permission to deviate therefrom may be granted by the state patrol. It shall be unlawful to display upon the front or rear of any vehicle, vehicle license number plate or plates other than those furnished by the director for such vehicle or to display upon any vehicle any vehicle license number plate or plates which have been in any manner changed, altered, disfigured or have become illegible. License plate frames may be used on vehicle license number plates only if the frames do not obscure license tabs or identifying letters or numbers on the plates and the plates can be plainly seen and read at all times. It is unlawful to use any holders, frames, or any materials that in any manner change, alter, or make the vehicle license number plates illegible. It shall be unlawful for any person to operate any vehicle unless there shall be displayed thereon valid vehicle license number plates attached as herein provided."

I never understood why you need 2 license plates? Maryland has 2 plates, but after your initial purchase and inspection, you never have to get the vehicle inspected again. Your fenders can be full of holes or flopping in the wind, bald tires or any mechanical problems are just fine. But you got to have a second license plate. Kind of seems the emphasis on the wrong thing to me.