On the ''classic bikes'' you'll find varying codes which don't always match to today’s ones e.g.
 
“ 2 3/4 x 18 “ or “ 2.75 x 18 “
· 2.50 = nominal tire width in inches
· 18 = nominal fixing diameter of rim in inches
 
The tire ''size'' is nowadays defined by a code found on the side e.g. 

120/70 R 17 M/C 58 H
· 120 = nominal tire width in millimeters
· 70 = nominal ratio (ratio between height / width in percentage: i.e 70% of 120=84mm)
· R = it reflects the kind of tire, here it means radius
· 17 = nominal rim fixing diameter in inches
· M/C = motorcycle
· 58 = load index
· H = max. speed index (in this case upto 210 km/h)
 
Other eventual elements found on tire could be ''DOT'' (Department of transport USA e.g. American homologation), four figures referring to production date (week/year), an arrow denotes wheel rotation etc..