I did a quick search on this subject on the forum, but couldn't really find an explanation. Why does West Virginia have fractional routes and how do they designate them?
When the State Road Commission took over county roads in the 1930s, the roads were divided into three numbered tiers based on importance. Fractional routes were the tertiary routes at the time. They were numbered with the numerator as the route they branched off of and the denominator as the number in sequence from the county line.
Please note that West Virginia's route numbering is fairly static. There are many cases now where fractional routes are actually major roads (like CR 19/24 in the Morgantown area), decommissioned routes were given fractional numbers (parts of old WV 73 are CR 73/73), or where whole numbers were renumbered to avoid conflicts with other routes (Randolph CR 33 renumbered to CR 37/8 after US 33 was extended into the county).