Flat-felled seam

What it is

Most seams join two pieces of cloth and leave two raw edges inside the garment, which then have to be finished so they do not fray. A flat-felled seam solves that in the seam itself: one seam allowance is trimmed, the other is folded over it, and the fold is stitched down through all the layers. The raw edges are enclosed. From the outside you see the join and one or two parallel rows of stitching beside it; from the inside you see a flat, folded ridge with no loose threads.

On a triple-needle felled seam the fold is sewn with three parallel rows at once, on a machine built for it. Two rows is the more common home for a workshop.

Why it was chosen

• Strength. The seam is held by two or three lines of stitching through four layers of cloth. It is the seam that takes the strain on the inside of a leg.
• No finishing. Nothing to overlock, nothing to bind, nothing to fray.
• It lies flat, which matters where a seam runs against skin.

Where it appears

The Standard Issue Jean's inseam — the seam running down the inside of the leg — is flat-felled with a 1.5 cm allowance. The outseam is not; it is a plain seam pressed open to show the selvedge, which is its own entry: The selvedge outseam. The Foundry Jacket's main seams are felled with three rows.

What it changes

Functionally: the seam is heavier and stiffer than a plain seam, and it uses more cloth — the allowance has to be wide enough to fold, and that is decided at the pattern. Visually: the rows of topstitching are one of the most recognisable things about a jean, and they are where the cloth fades first, because the ridge underneath rubs from inside. On a raw denim a felled inseam shows as two pale lines within a few months of wear.