Sometimes, you have to finish a seam allowance so it won’t fray. (Or, possibly, you’re like me and compulsively finish seams, whether they need it or not.) There are times when you can’t use a french seam, or you are working in an area too tight for a felled seam, and you want something nicer than an overcast edge. This method of finishing a seam allowance by hand will prevent them from fraying, and lightly reinforce the seam.
Here’s how you do it:
Cut down the seam allowance to 1/8".
With each stitch, you will catch 1 thread from the body of the fabric 1/8" from the seam, then pass the needle through the seam allowance just above the seam. This overcasts the seam allowance and secures it to the fabric.
Continue sewing in this manner, catching a single thread of the main fabric each time.
When you're finished, the seam is completely encased in thread and bound down. You can see the difference on the right, where the edge is bound, from the unfinished left side.
If you've been careful to catch only a thread at a time from the main fabric and not over-pulled your stitches, the finish is pretty invisible from the outside of the garment.