Topstitching

What it is

Most seams are sewn from the inside and hidden. Topstitching is sewn from the outside, through the seam allowances or a folded edge, and it shows. It is done in a heavier thread than construction thread, often in a contrasting colour, at a slightly longer stitch so each stitch reads clearly.

What it does

It is not decoration first. Topstitching holds a folded edge flat, keeps a pressed seam allowance from rolling, and adds a second line of stitching to a seam that carries strain. On a felled seam it is what holds the fold down. On a patch pocket it is the only thing holding the pocket on. On a waistband it keeps the layers from shifting.

That it also looks like something is a consequence, and a welcome one: the lines of stitching are most of what tells a jean apart from a trouser.

How it is done here

The stitching follows the seam at a fixed distance, guided by the edge of the presser foot or a guide screwed to the machine bed, so that the line is parallel and even. Two rows are sewn as two passes unless a twin-needle is used. Thread and needle are matched to the cloth; heavy denim needs a heavy needle to avoid skipped stitches. See Thread.

What it changes

Topstitching is where a garment shows its age first. The thread sits proud of the cloth and rubs against everything, so cotton and core-spun thread fades ahead of the cloth around it; and the ridge under a felled seam wears the cloth over it pale. On indigo denim the lines go from dark to white; on black they go grey. That is described under Fading and black denim. Broken topstitching is a small early repair and should be treated as one.