This I know something about
: clothing sizing
Like gardenerjoy said, designers and clothing manufacturers use "fit models" and not in every size. They cut up and down depending on the model's size.
My cousin's ex-wife has a fairly well known west coast clothing brand. We talked quite a lot about what she does as I have an abiding interest in fashion. Years ago she asked me if I wanted to be her fit model as she was branching out into plus sizes. If I hadn't been in AK at the time I probably would have done it.
MAK247, Depending on how long you've had those 15/16 pants, they very likely conform to the old "junior" sizing and will be roughly size 10 in modern sizes. Incidentally 15/16 started out meaning "15/16 years".
I owned/ran a vintage clothing shop for 4 years and I was sorely tempted to cut out those vintage sizing labels. The old size 12 roughly measures to and fits a modern size 4-6. Size 4/6's resisted trying a dress (normally) when they saw the size 12 label even though their eyes saw that it was roughly their size when they pulled it off the rack. Can you imagine the resistance in a modern size 10-12 to the vintage size 18 1/2 dress? Quite a problem that I tried to rectify by measurements and modern sizing on the shop tag. Still I had customers refuse to try on anything larger than their "size" dress.
Here's how this confusion happened: In the late 70s/early 80s manufacturers instituted new standards in clothing sizes in an attempt to standardize the industry, making it less brand dependent and to update sizes to consider the modern woman's height and concomitant weight gains over the decades, commonly called vanity sizing, but in fact while there is "vanity" sizing in the industry such as the 00 or sub-zero sizing, the standard changes did not start with that motivation.
Implementation of the new sizing changes started with the upper brands moving to mid tier brands followed by couture and mass produced clothing, To make things more confusing, sewing pattern manufacturers stuck by the old sizing years past the last ready to wear holdout.
Even today with international "standards" there is a fair bit of interpretation in the sizing which means you have to try it on. Being a thrift store fan I just try in the aisles over a tee and I re-donate if it doesn't work. I rarely have to do that though.
I'd say I'm a 1X, size 18W as an average. I found an adorable well made linen shirt very recently at a thrift that was marked 3X and it still had shop tags on it. It fit perfectly. It probably hadn't sold because it was mis-sized. I don't care. I've worn 3X before and it didn't affect my value as a person then, nor does wearing a mis-sized 3X make me fatter than I am now. I wish women's wear would go to a more man's type of sizing using measurements instead of an arbitrary number that women get so invested in.
My piece of history for the day.
Size?
My waist is 34 inches (size 18 on the chart) and my hips are 37 inches (size 6 on the chart). I'm actually wear sizes 10 and 11. WTH? 
