Pattern companies have, over the years, come up with various ways to help dressmakers fit different figures. One of these schemes was Simplicity's half-size patterns. If you look at vintage patterns, you run across them now and again. I'd noticed that the bust, waist, and hip measurements had slightly different proportions. But from the name "half-size" and the different measurements, one might reasonably assume that they were intended for women who fell in between the regular pattern sizes...
...but it turns out, that's not all. I discovered this reading 1940s and 1950s sewing books, like this one. It doesn't tell you all this on the pattern envelope, and if you don't really study and compare ALL the measurements, you might never know. Here's what i learned:
Most patterns are drafted to fit someone who's about 5'6" or 5'7" - that part i knew. Since i'm not that tall, i often have to check and adjust waist length, skirt length, sleeve length. But a half-size pattern is drafted with narrower shoulders, a little fuller hip, and a shorter overall height - to fit someone 5'3" or 5'4".
I've got a fair collection of books, from the 1910s to the 1970s (excepting the 1930s - have to look for something to fill in that gap). The good news is that you can now find a lot of this sort of thing reproduced online. There's Google Books, of course, but my first stop would be VintageSewing.info - it's a great resource. (For handy reference at the sewing table, though, it's hard to beat having a print edition in hand!)