State highways in Missouri were first designated in 1922 and Missouri's system was used (along with Wisconsin's) as a plan for the entire U.S. Highway system which was created in 1926. In Missouri, odd-numbered highways run north-south and even-numbered highways run east-west (with a few exceptions, such as Route 112). Missouri also maintains a secondary set of roads, supplemental routes, which are lettered rather than numbered.
Missouri has also changed highway designations with a US route or an interstate with the same number is designated through the state (MO-40 was redesignated Route 14 to avoid duplicating numbers with US-40 which also passes through the state).
In some states (such as Arkansas), highways are allowed to be discontinuous. Missouri overlaps highways in order to maintain continuity.
The Missouri Department of Transportation routinely uses the term "Route" in reference to the names of the roads. However, Missouri statutes define them as "State Highways". Missourians may use the terms "Route" and "Highway" interchangeably when referring to a state road.