Maryland has an extensive system of state highways, exclusive of the national Interstate and U.S. highway systems, that serves all 23 counties and the independent city of Baltimore, almost every incorporated city, town, and village, and most unincorporated places in the state. These highways are each designated Maryland Route X, where X is a number between 2 and 999. The highways are typically abbreviated MD X, although MD Route X and Route X are used less frequently. Because Maryland does not have a secondary route system or signed county route systems, all state highways are part of the main numerical system. That means the same set of numbers is used for both major highways and minor service roads, and almost every number has been used at one time or another.