WHAT IS IT?
A flat-felled seam is a strong, topstitched (page link) seam where the raw edges are enclosed. It’s also referred to as a felled seam or a flat fell seam. A true flat-felled seam will be cleanly finished on both sides of the garment. You may also see mock flat-felled seams, which have the appearance of flat-felled seams but are less work to sew. For flat-felled seams, you can choose to have the fell on the right side of your garment, which will have two lines of stitching, or on the inside of your garment, where only one line of stitching will show.
Inside of flat-felled seam
Mock flat-felled seam
WHEN DO YOU USE IT?
Use a flat-felled seam on heavy-duty clothing, such as work wear and jeans. It’s also great on tailored shirts that will be frequently washed or dry-cleaned. It would be an excellent seam finish on reversible garments and unlined garments, as it looks great from both sides. This type of seam finish is best on straight or mostly straight seam lines, as it’s a bit harder to sew flat-felled seams around curves. Keep it in mind for menswear, as there are more straight seams than curved ones.
HOW TO SEW A FLAT-FELLED SEAM
Sew the seam with a 5⁄8″ (1.5cm) seam allowance. If you want the fell to show with two lines of stitching, sew the seam with the wrong sides together. If you want only one line of stitching on your garment, sew the seam with the right sides together. Choose the side where you want the flat-fell stitching to go, and press the seams to that side.
Trim the seam allowance that’s on the bottom of the two to about ¼” (6mm).
Turn under the edge on the top seam allowance about ¼” (6mm) and press. Pin this edge in place, and edgestitch (page link) along the folded edge.
HOW TO SEW A MOCK FLAT-FELLED SEAM
Sew the seam right sides together with a 5⁄8″ (1.5cm) seam allowance. This type of seam will only work with the one line of stitching showing, with the mock fell on the inside. Serge (page link), zigzag (page link) or pink (page link) the edge of one seam allowance. Press the seams to one side, with the finished side on top.
Trim the seam allowance that’s on the bottom to about ¼” (6mm).
Flip the garment to the right side, and topstitch about 3⁄8″ (1cm) from the seam line, to hold down the finished edge.
Source : The Sewtionary An A to Z Guide to 101 Sewing Techniques + Definitions
About the Author : Tasia ST. Germaine