Take a look at your jeans or a men’s dress shirt and it's likely you’ll spot flat fell seams . On the outside of the garment there's a pair of stitch lines, while the inside is all tidy without any raw seam edges. If you've wondered how it's done, it's not magic — sewing a flat fell seam is a technique every garment sewer can (and should!) learn.
Flat Fell Seam
What You Need
Pin the fabric of the seam together, wrong sides facing.
Sew the seam at the project’s allotted seam allowance. (We used ⅝".)
Press the seam open on both sides of the fabric.
Trim one side of the seam allowance down to ¼". Do not trim the other side.
Press the untrimmed seam allowance over the seam and on top of the trimmed side. Fold the pressed side under, tucking the raw edge into the fold.
Press the fold, making sure the raw edge is fully tucked under and the flap left is the same width along the seam.
Line up your fabric in your machine so the stitching is close to the fold. Stitch along the edge of the fold, trapping all the raw edges inside.
That's it! You've just finished a neat and tidy flat fell seam.