Flatlocking is an ordinary overlock stitch with the tensions deliberately unbalanced so that the seam pulls open flat instead of standing up as a ridge.
Why it exists. A conventional overlocked seam sits proud on the inside of the garment. Against skin, under a waistband or inside close-fitting activewear, that ridge rubs. A flatlock seam lies level with the fabric, which is why sportswear and base layers use it.
How it is made. Sew the seam as a three-thread overlock with the needle tension loosened almost completely and the lower looper tightened. Then pull the two pieces of fabric apart firmly. The stitch spreads and the seam flattens out.
Two faces, two looks. Sewn with the fabric wrong sides together, the looper threads end up on the outside, giving a band of loops. Right sides together, the outside shows a ladder — two rails of thread with rungs between them. Both are decorative and both are used deliberately.
Practical points.
Trim little or nothing. The stitch has to spread, so disengage the blade or set a minimal cutting width. Fabric cut away cannot flatten out.
Use woolly nylon in the loopers for a soft, dense band that does not scratch.
It is not the strongest seam. With the needle tension almost off, it holds well on stable knits and poorly under real load. Do not use it on structural seams.
Test on scraps. The tension settings are specific to the fabric, and the pull-apart step is easier to judge by hand than to describe.
Questions people actually ask
My flatlock will not pull flat.
Needle tension is still too tight, or the blade trimmed too much away. Loosen the needle tension further and reduce the cutting width.
Which side is the right side?
Whichever you want. Wrong sides together puts the loops outside; right sides together gives the ladder effect. Decide before you sew, not after.
Last reviewed 2026-08-11