I have seen the pictures and have heard the descriptions of suit jacket construction, but nothing beats opening a couple up for yourself. I highly recommend a sartorisection for anyone interested in how they are put together, but especially if you do any thrifting. I had already felt that I had a pretty good idea of what I was feeling inside a jacket, but now I have confirmation and a very good in-person visual reference.
I went to the local Am-Vet store and picked out a half-canvas and full-canvas orphan jacket for $4 apiece; I then ripped out the lining using my Leatherman knife. On image-right, I left the sleeve lining in and only removed the main lining. On image-left, I removed the sleeve lining, pockets, and opened up the lapel all the way to the bottom of the jacket. This way I could easily see the canvas and fusing on the left, but I had the full construction for reference on the right. Since the backs are just plain, I scrunched them up so that the two armholes are right next to each other, in case that doesn't come across in the images.
Trying to feel around without grabbing a pocket can be tricky, so I wanted to keep a reference for how much of the front they take up. When feeling for the end of a half-canvas, I like to follow Jeffery Diduch's advice and use the area from the lapel and down, close to the edge of the jacket and where the front fabric is folded over to the inside.
In the first image, a Jos A Banks piece, the black fusing is easily visible down the entire length of the front, while the canvas stops a couple of inches above the first button. That's a bit high compare to others I have felt, some coming down much closer to the top button.
In the second image, a Hickey Freeman mainline, you can see the canvas running down the full length and no fusing in the front. You can also see the pad stitching in the lapel. The collar is also much more interesting than that of the JAB.
There were no surprises for me, as this confirmed what I already felt confident about, but it was still a worthwhile learning experience. I very much like to have a hands-on quality to my education, and this was similar to the first time I cracked open a car engine.