Montana's state highways are classified as either primary or secondary.

[1] The square markers used today to identify primary Montana highways are only slightly different from their 1940s and earlier predecessors.

The old markers used the word "ROUTE" above the number in big size, the route number in the middle, and the word "MONTANA" from edge-to-edge at the sign bottom.

[1] S route designations first appeared on the state highway map in 1960[5] and are abbreviated as "S-nnn".

[1] (Early markers were white numbers on black arrowheads with the word Montana in the flat top of the inverted arrowhead and Secondary appearing below the route number on the shields.)