The Idaho Territory was organized in March 1863, and Owyhee County was the first county in the territory to be organized, in December of that year.

Idaho's present-day boundaries were established in 1868, and Lemhi County was created the following year.

By the time Idaho was admitted to the Union as the 43rd state in 1890, a further eight counties had been created, bringing the total to 18.

For instance, the four counties beginning with 'L' in alphabetical order are Latah, Lemhi, Lewis and Lincoln; the codes for these counties are thus 1L, 2L, 3L and 4L respectively.

Elmore, Idaho, Kootenai, Nez Perce, Shoshone, Valley, and Washington Counties are the only ones in the state with their respective first letters; in these counties, the letter alone serves as the code.