About the Mississippi State Highway System
One and two digit state routes are primary and orientated with the lowest odd numbered route running north-south from west to the east and the even numbered routes assigned to east-west highways and counting upwards from north to south. There are some exceptions.
Mississippi also utilizes a route clustering system for the numbering of three-digit routes. These range from the 300 series at the northern end of the state to the 600 series within the Gulf Coast counties. 100 series numbers are reserved for former alignments of U.S. Highways and act as business loops or alternate alignments to the parent US Route. Mississippi Highway 245 is an exception to this rule as it represents an old alignment of U.S. 45 Alternate.
Unsigned highways are legislative in nature and range in the 700s, 800s and 900s, with a clustering system from north to south similar to the 300-600 series. Signs for these routes appear occasionally throughout the state.