Plain stitches fall into four general categories&colon; chainstitch, lockstitch, overlock, and coverstitch. Like lockstitch and chainstitch, coverstitch can be formed anywhere on the material being sewn. A zigzag stitch is variant geometry of the lockstitch. A zigzag stitch is a variant geometry of the lockstitch. A lockstitch is the most common mechanical stitch made by a sewing machine. The first American lockstitch sewing machine was invented by Walter Hunt in 1832. Most industrial lockstitch machines sew only a straight line of stitches. The term "single needle stitching", often found on dress shirt labels, refers to lockstitch. A better stitch was found in the lockstitch. There are different types of lockstitch industrial machines. In addition to the normal lockstitch, construction stitches include edgestitching, understitching, staystitching and topstitching. The lockstitch uses two threads, an upper and a lower.