Ply Ratings...

    Back in the old days (until say mid 1950s), tires were made with
cotton canvas belts run on a bias (at an angle) across the tread area
(hence "Bias Ply" tire)..

    The more canvas belts, the stronger the tire and larger weight
carrying capacity...

    Nowadays with the advanced materials (Kevlar/aramid, polyester, and
steel) they can get the same load rating and strength with fewer
layers/plys...

    not uncommon to see a 4 ply rated tire with one side wall and two
tread plys...

        JonR