English: A mainspring in a Sea-Gull ST16 wristwatch movement. This watch is "automatic" or self-winding, which means that there is an eccentric weight mechanism in the watch which winds the spring using the motion of the owner's wrist. To prevent the mainspring from being overwound, which could result in the mainspring breaking, the spring is attached to the barrel which contains it with an expansion spring "bridle" device. When the watch is fully wound, further winding pulls the end of the mainspring bridle loose and it just slides along the barrel wall, so the mainspring can't be broken by overwinding. This photo is taken through the face of a skeleton watch, as seen in File:Automatic skeleton watch.jpg. A similar spring uncoiled is seen File:Mainspring Chinese uncoiled.jpg