I've been asked to talk about Ply Ratings. First, the technical background.
As a tire gets larger, it takes more strength to contain the air pressure (or whatever gas is being used as an inflation medium). Those of you of an engineering nature should look up "Thin Walled Pressure Vessels". Here's the Wikipedia entry on the subject: https://en.wikipedia.org/wiki/Cylinder_stress
In the old days, cotton fabric was used as a reinforcement, and to get more strength, more layers (plies) of it were used. When the tire industry started to standardize (maybe about the early 1900's), it became apparent that a standard measure was needed - thus the invention of the "Ply Rating" (PR), where a standard strength was assigned to a ply, and since all tires were bias ply tires, they came in pairs - 2, 4, 6, etc. It is opened ended with no limit. In very very large tires, the PR's exceeded 60 PR.
The idea of PR is that in order to use more inflation pressure, the tire has to be stronger - hence more plies.
Side note: Sharp thinking folks will realize that as tires get physically larger, there reaches a point where the tire has to jump up an increment in PR in order to contain the same pressure as a physically smaller tire. You can see a bit of this in Light Truck tires, but in very very large tires (like mining tires), it is quite apparent.
I think it is obvious that using the term "Ply Rating" has a built in confusion factor - the actual number of plies versus the ply rating. Not to mention that a ply can be made out of many different materials, and those materials do not have the same strength. For example, steel wire can be used as a fabric and it is obviously very strong compared to cotton.
To eliminate this confusion, PR was discontinued in favor of "Load Ranges" (LR) - A, B, C, D, etc., where A = 2 PR, B = 4 PR, C = 6 PR, etc.
EXCEPT: In passenger car tires, the use of Standard Load and Extra Load were substituted. Unfortunately, it is NOT an exact fit. A Standard Load tire is not exactly a 2 PR and an Extra Load in not exactly a 4 PR, particularly when it comes to larger tire sizes (See my side note above!)
I'm going to guess the transition from PR to LR took place in the late 1960's, but it is an American invention, and the Europeans don't quite subscribe to this an prefer their invention of "Load Index" Load Index has a nice property to it as it takes care the the problem of the jumping increments mention in the side note above.
I really dislike the use to the term "Ply" when people talk about the strength of a tire. "Ply" doesn't tell me how strong a tire is. For example, a typical over the road truck has a single body ply (made of steel wire) with 3 or 4 steel belts (only 2 of which are actually needed!) for a Load Range H, (the equivalent of a 16 PR).
Another example is an LT tire. Typically they have 2 body plies of polyester with 2 steel belts - with perhaps 1 or 2 cap plies (made of nylon) - REGARDLESS of Load Range! But passenger car tires can also have the same number of plies (not always!). It all depends on what material is used and how much of it there is and what is required strengthwise.
May I suggest that folks try to stop using the term "ply" when they mean "Ply Rating" - AND - they discontinue the use of the term "PR" as it leads to confusion (as we can see in this thread!)