Flat Felled Seams are great because they are strong, there is no edges to ravel or to rub on your skin and they can make a garment reversible. Below are the basic steps to creating a flat felled seam.
3. Iron the seam allowance to one side, so that the longer allowance covers the narrow allowance.
This is a beautiful and classic seam finishing. I have a silk charmeuse camisole that uses this finish to allow it to be reversible.