The casing of a diagonal ply tyre consists of a number of rubberised cord plies with edges wrapped around the bead wires. (The bead ensures that the tyre sits firmly on the wheel’s rim). The number of plies determines the load capacity of the tyre.
Cross-ply tyres for passenger cars generally had between two and six rayon or nylon cord plies. Even today, van tyres are said to have 6 or 8 PR, with a higher number of plies meaning a greater load-carrying capacity.
Cross ply tyres are nowadays few and far between on modern cars, having been replaced by the more common radial construction.
Diagram shows a typical cross ply tyre construction