Sewing Method, a variation on the Hong Kong/Flat Felled/French seam.
I have seen it in a book years ago for making some outdoor camping stuff, might have been a seam on a tent or tent cover. Don’t remember the usage, but I modified the size for use in making men’s clothes.
The finished width is twice the seam allowance you used, for this example, I use 1/2″ seam allowances, finished width is 1″, this works best on light to medium/heavyweight, does not come out so well with heavy to very heavy weight fabric. I use it with medium bottom weight canvas/cotton fabric for men’s shorts and pants.
I have used in jackets, pants, and shorts (mostly cargo), the outside seam. I know most use the traditional flat fleed seam for that, but I like completely symmetrical seam when the fabric has equal layers on each side of the seam, a flat fled has them all pressed to one side.
Here the video on how to make it, I can’t even begin to write out the written direction to do it.
Here is a bunch of examples of the different looks you can get with it.