Looper

A looper is the part of the merrow machine or serger that replaces conventional bobbins. One or two loopers form thread loops that the needles pass through to complete the stitch.
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A standard sewing machine, a lockstitching machine, uses a bobbin and top thread to complete the stitch. The serger, or merrow machine, uses top thread(s) and bottom looper(s) to complete the stitch. Thread specially developed for the high speed of the serger machine works best in the loopers. Some machines also afford the use of larger, non-traditional threads in the loopers for decorative effects.