flatlock

A flatlock seam is a stitch construction in which the seam allowance is folded and stitched so that it sits flat and flush against the garment body rather than projecting outward. The technique eliminates the loose, bulky edges typical of conventional seams. This is achieved by folding one or both seam allowances to lie parallel with the fabric face, then securing them with a row of stitching that catches both the fold and the adjacent fabric layer. The result is a smooth, compact seam with no protruding thread edges.

Flatlock seams appear in two primary configurations. In single flatlock, one seam allowance is folded under and stitched down. In double flatlock, both allowances are folded toward each other and locked together with stitching between them, creating an even more compact result. The technique works best on knit fabrics and stretchy materials, where the flexibility accommodates the folded layers without bunching. It is less commonly used on rigid wovens, where excess fabric layers can create visible ridges.

Common applications and variations

Flatlock seams are standard in swimwear, athletic wear, and other performance garments where reduced bulk improves comfort and reduces chafing. They are also used in lingerie and high-end casualwear where aesthetic seam lines matter. The seam creates a decorative effect in some applications, particularly when the stitching is visible on the outside of the garment; contrast thread can emphasize the flatlock line as a design feature rather than merely a functional seam.

The main drawback of flatlock construction is time: folding and positioning the layers requires extra handling and precision. Automation through specialized machines with folder attachments has made the technique more economically viable at production scale. When done incorrectly, folds can twist or wrinkle, leaving the seam visibly uneven. On fine fabrics or delicate fibers, the extra stitching passes can leave visible needle marks or damage the material.

The name derives from the appearance: the seam lies flat rather than standing proud, and the layers are locked together by the stitching. The technique sits between simple seam finishing and more complex construction methods like binding or piping. In apparel manufacturing, flatlock is considered a mid-to-premium construction detail, signaling durability and comfort focus rather than basic assembly.