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From 1935 until around 1982, Washington license plates were coded by county. A one- or two-letter prefix denoted the county of origin, and letters were assigned based each county’s registered vehicle population as of the early 1930s, with King County, the most populous, receiving letter A, followed by Pierce with B, and so on. After letter Z, the 13 smallest counties received two-letter prefixes derived from the county name. These codes were unchanged through 1957.

For the first six years, the passenger vehicle class was the only license plate type to receive a county code.  Starting in 1941, truck and trailer formats were adapted to include county codes. In 1954 the system and codes remained the same, except the county code became a suffix. 

With the adoption of a modern ABC 123 numbering format for passenger vehicles in 1958, the county codes were modified to a three-letter format as shown below (county coding was discontinued for truck and trailer plates).  By 1981 the system became too complex to manage and the state discontinued the practice.

County	Seat	1935-1957	1958-1981
King	Seattle	A	AAA-AZZ, OAA-OZZ, IAA-IZZ, UCA-UKZ
Pierce	Tacoma	B	BAA-BZZ, TBA-TFZ
Spokane	Spokane	C	CAA-CZZ, SLA-SMZ (excl CLx, CMx, CUx)
Snohomish	Everett	D	DAA-DVR (excl DSA-DUZ)
Yakima	Yakima	E	EAA-EKZ
Whatcom	Bellingham	F	FAA-FFP
Clark	Vancouver	G	GAA-GND, HHA-HHZ
Grays Harbor	Montesano	H	HAA-HDP
Kitsap	Port Orchard	I	KSA-KYZ
Thurston	Olympia	J	JAA-JEE
Chelan	Wenatchee	K	KAA-KDC
Lewis	Chehalis	L	LAA-LDG
Skagit	Mount Vernon	M	MAA-MDS
Cowlitz	Kelso	N	NAA-NFR
Walla Walla	Walla Walla	O	WWA-WYP
Whitman	Colfax	P	PAA-PCF
Clallam	Port Angeles	Q	CLA-CMZ, PDA-PDF
Benton	Prosser	R	RAA-REU
Kittitas	Ellensburg	S	SAA-SBH
Lincoln	Davenport	T	TAA-TAN
Okanogan	Okanogan	U	UAA-UBJ
Pacific	South Bend	V	VAA-VAW
Stevens	Colville	W	WAA-WBC
Mason	Shelton	X	XAA-XAZ
Jefferson	Port Townsend	Y	YAA-YAM
Klickitat	Goldendale	Z	ZAA-ZBD
Adams	Ritzville	AD	YYA-YYR
Asotin	Asotin	AN	ZYA-ZYL, ZZA-ZZZ
Columbia	Dayton	CO	CUA-CUG
Douglas	Waterville	DO	DSA-DST
Franklin	Pasco	FN	FNA-FSF
Ferry	Republic	FY	FYA-FYE
Garfield	Pomeroy	GA	GDA-GDF
Grant	Ephrata	GT	GTA-GVW
Island	Coupeville	IS	1958: CVA-CVL   1963 up: ISA-ITH
Pend Oreille	Newport	PO	PEB-PES
San Juan	Friday Harbor	SJ	SJA-SJG
Skamania	Stevenson	SA	SKA-SKJ
Wahkiakum	Cathlamet	WA	WMA-WMF
Dept. of Licensing Headquarters	JJ	JJA-JJZ, XXA-XXZ
Dept. of Licensing Headquarters (1939-1942)	LD

Recap:

1935-1981 Passenger vehicles issued and coded by county

1941-1957 Truck and Trailers were issued and coded by county

No other vehicle types ever received county-coded license plates. From 1958 on, only passenger vehicle license plates (with ABC 123 serial format) were county coded.