Further to the earlier answers, the Senior TT WAS once a World Championship event. So were the Junior TT (350cc), the 250cc TT and the 125TT. I don't know when it all changed but that's how it was in the 1950s when Geoff Duke was king of the 500s (Norton then Gilera), Bill Lomas the 350s (Moto Guzzi) and Werner Haas the 250s and 125s (NSU). John Surtees subsequently took over the 500cc mantle on the MV after Ray Amm was killed in Italy on his first ride for MV.