Overlock Stitch

Seaming, Trimming and Finishing in One Pass

The overlock machine, widely known in home sewing as a serger, does three jobs simultaneously that a lockstitch machine does separately or not at all. Fabric enters under the presser foot; a knife, positioned just before the needles, trims the edge to a clean, exact line; needles and loopers then form stitches that pass through the fabric and loop around its freshly cut edge. The seam that emerges is joined, its allowance is trimmed to a uniform width, and its raw edge is wrapped in thread so it cannot fray. Inside almost any modern T-shirt, sweatshirt or pair of leggings, that wrapped, slightly springy seam is overlock work.

The stitch is formed mainly by loopers rather than a bobbin. Overlock machines have no bobbin; instead, upper and lower loopers carry threads around the edge and interloop them with the needle threads. Machines are described by their thread count: a three-thread overlock gives a standard edge finish, a four-thread adds a second needle for a stronger seam with a safety stitch line, and simpler two-thread and rolled-hem settings produce narrow finishes for lightweight fabrics and decorative edges. Because the threads loop rather than lock tightly, an overlock seam has built-in stretch, which is why overlock became the standard construction stitch for knitwear: the seam can extend and recover with the fabric instead of snapping its stitches.

Overlock in the Factory and at Home

In garment factories the overlock machine is a production staple, dividing work with the lockstitch machine roughly evenly across a typical factory floor. Lockstitch handles flat, precise seams and topstitching; overlock handles the fast joining and finishing of panels, especially in knits and in the inside seams of woven garments, where a separate seam and a separate edge finish would double the handling. Specialised overlock variants attach elastic, sew mock safety stitches on shirts and jeans, and finish the edges of everything from lingerie to upholstery.

Home sewers adopted the overlocker, as the serger, later, and it changed home dressmaking in one specific way: knit fabrics became easy to sew professionally. A home overlock machine seams, trims and finishes a jersey seam in seconds, with the same inside appearance as bought clothing. Threading several loopers is more involved than threading a lockstitch machine, and the knife that makes the finish so neat also removes any margin for error, since excess cloth is cut away as the seam is sewn. Patterns for overlock construction therefore allow narrower seam allowances than lockstitch patterns.

The Coverlock Variation

The coverlock extends the family from edges to hems. A coverlock can do hems in the middle of a cloth, away from any edge, by having the detachable knife removed: nothing needs trimming in the middle of a hem, so the machine simply lays its covering stitches, the parallel lines of topstitching with looper thread beneath that finish the hems of T-shirts and activewear. Combination machines switch between overlock and coverlock functions. For other sewing-machine stitches, the lockstitch page gives the contrasting two-thread construction, and together the two stitches account for the great majority of seams in modern clothing.