Underarm gussets and sideseams

A quick post, so please ask if it isn't clear - mainly photos to illustrate the last post, on underarm gussets and sideseam gussets.

Right.

You put in the square underarm gussets sewn so that one side is seamed to the body-front immediately below the sleeve; the adjacent side is sewn to the sleeve-front; the next side is sewn to the sleeve-back and the final side is sewn to the body-back.

First three pics show this at various stages - the last one shows an antique shirt of mine which is from around 1780-1820 I think. It has a smaller underarm gusset but exactly the same principle. Although a lot neater than mien are...

Then sew the side-seam as a felled seam (ALL seams are felled, you should have no raw edges anywhere).
Leave a split at the bottom, which you hem each edge of. You can add a triangular gusset to this as well if you like, partly to prevent the seam splitting further up and partly to help hide the bodge-point that I always seem to have! I haven't done this yet on the shirt I'm making, but here is the 1780s one's side-seam and gusset. A square folded diagonally and just fitted round the point where it changes from felled side-seam to two hemmed edges.

The large-focus pic is the outside and the close-up is the inside. I've added an overview just for the sake of it!
You can see how the seamstress two centuries ago cleverly offset the square gusset a tiny bit so more of it is on the inside and only the tiniest tip of one corner is visible outside for neatness's sake.

The diagram is just a quick one - to change from side-seam to hemmed-edges one piece just goes from being folded under (shown in green) to being flded under twice, very simple.
The other, shown in red, has to go from being folded upwards to being folded under twice, so I usually make a small snip with scissors.