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Washington license plates were coded by county for nearly fifty years.
Beginning in 1935, passenger vehicles were assigned a one- or two-letter prefix that denoted the county of origin, with letters assigned based each county’s registered vehicle population as of the early 1930s. King County, the most populous, received 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. This practice expanded to truck and trailer plates in 1941 (lasting through 1957). 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 in Washington. From 1958 on, only passenger vehicle license plates (with ABC 123 serial format) were county coded. Letter suffixes and prefixes on other license plate types (such as dealers from any year, or any non-passenger class types from 1958 and later) are often erroneously identified as county codes.
