The French Seam is my favorite seam of all time. It’s fully encased and leaves no visible stitching on the right side of the garment. Any time I have to make an unlined garment, you can bet I’m using french seams (or some variant thereof).
Width:Variable, but 1/4″ is common.
Seam Allowance Required: 1/2″ for 1/4″ finished seam, or twice the desired finished seam.
Note: I very frequently combine these two steps by serging. I like my seams extra strong!