A very fine spring (also called a "hair spring") in a mechanical watch that returns the balance wheel back to a neutral position.

Component of the regulating organ (s.) that, together with the balance (s.), determines the movement's precision. The material used is mostly a steel alloy (e.g. Nivarox, s.), an extremely stable metal compound. In order to prevent the system's center of gravity from continuous shifts, hence differences in rate due to the watch's position, some modifications were adopted. These modifications included Breguet's overcoil (closing the terminal part of the spring partly on itself, so as to assure an almost perfect centering) and Philips curve (helping to eliminate the lateral pressure of the balance-staff pivots against their bearings). Today, thanks to the quality of materials, it is possible to assure an excellent precision of movement working even with a flat spring.

Fine spiral spring that returns the balance to equilibrium and which maintains equal and independent oscillations irrespective of their amplitude.

The balance spring is a device used in a mechanical watch to control the vibrations of the balance wheel.