South Dakota is one of a small number of states that use numeric prefixes in license plate numbers to identify the county that issued the plates. The present-day prefixes have been used since 1987. They were also used during the years 1956-1975 (but not during the intervening years 1976-1986).
The nine most populous counties in 1956 were assigned numbers 1-9, beginning with Minnehaha County (county seat: Sioux Falls), the most populous. The other counties—arranged alphabetically—received codes 10 through 64.
During the years since 1956, three additional counties have been assigned numbers 65-67:
- Shannon County (adjacent to and east of Fall River County) is one of two counties with no county seat. Fall River County's county seat, Hot Springs, serves as Shannon County's administrative center.
- Washabaugh County was dissolved on January 1, 1983 and merged with neighboring Jackson County on the north. Apparently Bennett County (to the south) continues to issue plates with prefix 66 to vehicles owned by residents who live in the area that was previously Washabaugh County. (Prefix 66 isn't currently represented by South Dakota PlateFinder.)
- Todd County (adjacent to and west of Tripp County) is the other county with no county seat. Winner, the county seat of Tripp County, serves as Todd County's administrative center.