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An overlock stitch connects two pieces of fabric by using a series of thread loops that wrap around the outer edge of the material to prevent it from fraying. A seamstress may use this stitch for hemming, reinforcement, or decorative effect due to its strength and the fact that it is visually obvious. This type of stitching is sometimes called serging, overedging, or merrowing.
Usually, an overlock stitch is recommended for stretch fabrics, where using a standard machine stitch can cause pulling or bumps along the seam. The stitches, in addition to being strong, are stretchier than those made by a standard sewing machine. This is often the most effective way to finish the seams of garments made from jersey, Lycra®, and spandex. The stitch also works well on garments that get a lot of wear — such as T-shirts or sweatpants — and garments made of knit fabrics, such as sweaters. Many commercially made garments are finished this way.
True overlock stitching requires an overlock machine, or serger. Unlike a regular sewing machine that uses a single bobbin to feed thread to its needle, an overlock machine pulls thread from several different spools (called cones) to create loops of thread. The machines use several needles — as many as five at a time — and work quickly. Also, each machine usually trims threads as it stitches so that the garment is finished when it is removed from the machine. It is, however, possible to mimic the stitch on a standard sewing machine by using a ballpoint needle and sewing a zig-zag stitch directly along the edge of the fabric.
The first overlock sewing machine was invented in the 19th century in the United States. Its creation came about during the industrial boom of textile production in the northeast part of the country. Most overlock machines are made overseas and tend to cost more than standard sewing machines.
There are several ways to vary the appearance of an overlock stitch. The length a person chooses for stitches will affect the way the stitch looks — the longer the stitches, the more open and visible they will be. Shorter stitches appear more solid and compact. The thread count also contributes to the appearance of the stitch. Using one thread creates an open, lacy stitch, while using four or more creates a thick, more solid stitch appearance.