Editing Lockstitch (section)

==Structure==
The lockstitch uses two [[Thread (yarn)|threads]], an upper and a lower. Lockstitch is named because the two threads, upper and lower, "lock" (entwine) together in the hole in the fabric which they pass through. The upper thread runs from a [[Bobbin|spool]] kept on a spindle on top of or next to the machine, through a tension mechanism, through the take-up arm, and finally through the hole in the [[Sewing needle|needle]]. Meanwhile, the lower thread is wound onto a [[bobbin]], which is inserted into a case in the lower section of the machine below the material.

To make one stitch, the machine lowers the threaded [[Sewing machine needle|needle]] through the cloth into the bobbin area, where a [[rotary hook|rotating hook]] (or [[bobbin driver|other hooking mechanism]]) catches the upper thread at the point just after it goes through the needle. The hook mechanism carries the upper thread entirely around the bobbin case so that it has made one wrap of the bobbin thread. Then the take-up arm pulls the excess upper thread (from the bobbin area) back to the top, forming the lockstitch. Then the [[feed dogs]] pull the material along one stitch length, and the cycle repeats.

[[Image:Sewing.ThreadTensionProblem.jpg|thumb|right|200px|Correct and incorrect thread tension]] Ideally, the lockstitch is formed in the center of the thickness of the material — that is, ideally the upper thread entwines the lower thread in the middle of the material. The thread tension mechanisms, one for the upper thread and one for the lower thread, prevent either thread from pulling the entwine point out of the middle of the material.

Prior to the invention of the rotating hook, lockstitch machines placed the lower bobbin inside a miniature [[Shuttle (weaving)|shuttle]] which would be passed through the loop formed when the needle passed through the fabric and then began to retract again.