

Overlock thread is used for finishing, or overlocking, heavily frayed fabrics, as these would otherwise fray into the seam after several wears or washes, resulting in visible holes in the seams. It is thinner than regular sewing thread, yet very tear-resistant.
Because overlock and cover stitches require much more thread than those sewn with a home sewing machine, overlock threads are sold commercially on large cones. There's also a reason why the thread itself is finer than a standard all-purpose thread: The seam is worked with up to four threads. A thicker thread would result in a bulging seam. This feels uncomfortable when wearing the garment. And it doesn't look good either, especially on tight-fitting garments.
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