IIRC, leather was originally measured by the weight per square foot. The problem with that is that the moisture content of the leather can vary considerably. So the "weight" was standardized at 1 oz = 1/64 in. Thus 8 oz leather should be 1/8" thick. But it is very difficult to have a side of leather to be shaved evenly from shoulder to belly. So, leather is generally given a rating range. Thus: 8-9 oz., 6-7 oz., etc. I have found it best to actually measure the thickness, as sometimes, whoever grades the leather can be a bit off. I generally use Tandy veg-tanned leather, and have sometimes found what is called "6-7 oz" may be 7-8 oz or 5-6 oz. For that reason, I use a vernier calipers on the shoulder area when selecting individual sides.