What is a flat felled seam?
A flat felled seam is a topstitched seam application that fully encases the seam allowance edges for a clean finish on both the right and wrong side of the seam.
The most common instances you'll find flat felled seams being used is on denim garments like jackets and jeans, as well as button down blouses (especially men's styles). This seam finish not only provides a clean look on the wrong side of the garment, it is also extremely durable which is much needed for these heavy duty clothing items. From a design stand point, the topstitching used with a flat felled application works perfectly with denim and other similar styles, as it provides the casual feel these designs are built for.
The secret to sewing a flat felled application is the grading of the seam allowance layers. Grading simply means trimming one of the seam allowance layer shorter to eliminate bulk and facilitate the folding and stitching process. The narrow trimmed seam allowance edge is eventually enclosed by the longer, untrimmed layer and topstitched to complete the flat felled application.
Perhaps the biggest difference between sewing a conventional seam and a flat felled seam is that the seam edges are first aligned and stitched with fabric wrong sides touching. Once stitched, the rest of the flat felled application steps happen on the seam's face side. In conventional applications, the seam finishes are applied on the seam's wrong side exposing a clean seamline on the garment's face side. A flat felled seam is quite the opposite- the applications face side features two topstitches, while its wrong side has a clean seamline (with a topstitching along one side).