Before load range charts, ply ratings were used to reveal the relative strength of tires. Ply rating was done by building up multiple cotton fabric layers in the tire casing, with the layers placed in a way that the threads in each layer were at an angle to each other. This was called a bias arrangement, which allowed tension to be distributed equally throughout the fabric layers.
Counting the number of cotton layers that the tire could handle revealed its ply rating. A higher ply number indicated that the tire's construction was built for heavier duty application.
Ply ratings didn't have any odd numbers because at least two plies were needed to create a bias arrangement. When an old tire was stamped with a 20-Ply rating, it meant it had the strength of 20 layers of cotton fabric.
Eventually, cotton was replaced with nylon because the latter was a stronger material and only a few layers were needed to identify a tire's ply rating. That's why some tires are stamped with the words “2 Ply/4 Ply Rating,” which meant the tire's strength was equivalent to two nylon plies or four cotton plies.
As ply materials continued to improve with the introduction of radial construction and steel ply materials, old ply ratings eventually lost their importance. This resulted in the newer tire strength identification used today, which is load range.