The lockstitch uses two threads, an upper and a lower. The upper thread runs from a spool kept on a spindle on top of or next to the machine, through a tension mechanism and a retracting arm, and finally through the hole in the needle. The lower thread is wound onto a bobbin, which is inserted into a case in the lower section of the machine. To make one stitch, the machine lowers the needle through the cloth into the lower section, where a hook catches the upper thread at the point just before it goes through the needle. The hook mechanism loops the upper thread entirely around the bobbin case, so that it has made one wrap of the bobbin thread. Then the retracting arm pulls the excess upper thread back to the top of the machine and the feed dogs pull the cloth back one stitch length.
Lockstitch is so named because the two threads, upper and lower, "lock" together in the hole in the fabric through which they pass. Unlike chain stitch, lockstitch does not unravel easily, making it difficult to remove. The term single needle stitching, often found on dress shirt labels, refers to lockstitch, as opposed to chain stitch which unravels easily and is usually used on lower quality garments.
Two other common kinds of mechanical stitches are overlock and chain stitch.