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The U.S. state of Washington first required its residents to register their motor vehicles and display license plates in 1916.
Passenger baseplates 1916 to present
In 1956, the U.S. states and Canadian provinces came to an agreement with the Automobile Manufacturers Association that fixed the size for license plates for vehicles, except those for motorcycles, at six inches in height by twelve inches in width, with standardized mounting holes. The first Washington license plate that complied with these standards was a modification of the 1954 plate, introduced in January 1956.
References
Vehicle registration plates of Washington (state) Wikipedia(Text) CC BY-SA