A Lockstitch

A lockstitch is a type of stitch made by a sewing machine. It is used to sew fabric or leather. The stitch can be used to attach pieces of material or as a decorative top stitch. It consists of a top thread and a bottom thread that cross each other and lock at every point where the needle enters the material. The lockstitch is versatile and has several variations that make it one of the most common stitches in machine sewing.

The mechanism of creating a lockstitch is simple but effective. The threaded needle of the sewing machine enters the material. The needle pushes below the material, carrying the thread with it. Underneath the material, the bobbin thread is looped around the section of thread carried by the needle. As the needle rises back out of the material, the looped threads are pulled tight and locked.
The advantage of lockstitches is that they will stay secure if the stitch line is cut. Chain stitches and most serger stitches, for example, will unravel if the stitch line is cut. Lockstitches will not come apart if a loose thread or end thread is pulled. By contrast, pulling a loose thread on a chain-stitched seam can cause the entire stitch line to unravel and the material to come loose.
There are a wide variety of lockstitch styles. The tension on both the needle thread and the bobbin thread can be adjusted tighter or looser. The length of the stitches can be made short or long. Many home sewing machines have the ability to sew decorative lockstitches, with the movement of the needle creating designs and patterns.
A zigzag stitch is a variation of a lockstitch. When a sewing machine sews a zigzag stitch, the needle moves back and forth in a preset width. At the end of each sideways movement of the needle, the needle enters the material and makes the outside points of the zigzag stitch. The threads are locked at each point of the zigzag. The width and length of zigzag stitches are adjustable, allowing for a wide variety of stitch designs.
Lockstitch sewing machines and closely related double-needle sewing machines are available in both the home and commercial sewing machine markets. Double-needle sewing machines create two parallel rows of lockstitches, with each needle being met by a separate bobbin. Although they are primarily in the commercial market, there are some smaller versions that are made for the home market.