I just can't help but respond to this mini-thread from a few days ago...
When a tire is said to be '6-ply" or "8-ply', etc., it refers to the
sidewall. However, it is not correct to say "6-ply sidewall" - you must say
"6-ply RATED" sidewall. Modern tires for our vans in most cases have only 2
physical sidewall plies. However, based on the construction of those plies,
it may carry a ply rating of 6, 8, etc. 6-ply rated tires would correspond
to Load Range C, 8 to D, 10 to E, etc. As Mike F. alluded to,  the origin of
the ply rating is from days gone by when the sidewalls actually did have say
6 cotton plies and adding more plies was how strength was increased. It's
basically an archaic rating system. Just in case you were wonderin' (Info.
courtesy of Jim at Century Tire)

Doug Fitz-Randolph
Yarmouth, ME
'90 Syncro
(Goodyear Wrangler HT Load Range D 8-ply rated tires with 2 polyester
sidewall plies)