Prairie County is a county in the U.S. state of Montana. As of the 2020 census, the population was 1,088, making it the fifth-least populous county in Montana. Its county seat is Terry. Prairie County was created by the Montana Legislature in 1915 out of parts of Custer, Dawson, and Fallon Counties. The name was selected in a contest and reflects the predominant landscape of the region.

Custer County is a county located in the U.S. state of Montana. As of the 2020 census, the population was 11,867. Its county seat is Miles City. The county was established on June 2, 1865, as one of the nine original counties of the Territory of Montana named Big Horn County, and was renamed on February 16, 1877, in honor of Lieutenant Colonel George Armstrong Custer.