Definition
A flat-felled seam is a strong, fully enclosed seam in which one seam allowance is wrapped around the other and stitched flat, showing two parallel rows of stitching.
Flat-felled seams are what you see on the inside leg seams of jeans, on men's shirts and on workwear. They are durable, lie flat against the skin and look finished inside and out, which makes them good for unlined jackets and reversible garments.
A common method, with a 1.5 cm (5/8 in) allowance:
- Sew the seam wrong sides together, so the fell ends up on the outside, or right sides together for a fell on the inside.
- Trim one seam allowance to about 6 mm (1/4 in).
- Fold the raw edge of the wider allowance under, wrapping it around the trimmed one, and press both flat toward the trimmed side.
- Edgestitch along the folded edge through all layers.
The finished fell is typically 6 to 10 mm (1/4 to 3/8 in) wide. A felling foot folds and guides the fabric for you, and a slightly longer stitch looks crisper.
Flat-felled seams work best on straight or gently curved seams in medium-weight wovens like denim, chambray, linen and twill. On thick fabric, grade the layers to reduce bulk. They are hard to sew on narrow tubes such as sleeves once closed, so plan to fell those seams while the piece is still flat.
For lighter fabrics, a French seam is easier; other options are compared under seam finish.