English: An uncoiled mainspring from a Seagull ST16 watch movement, which is an automatic (self-winding) movement. The mainspring is attached to a circular steel expansion spring, often called the "bridle" (seen on the top left), which presses against the inside wall of the barrel, which has serrations or notches to hold it until the bridle deliberately slips to prevent over tensioning the mainspring. This spring is seen coiled in the same style of movement in File:Watch automatic mainspring.jpg.