One of the most important components of a mechanical timepiece (same as “balance spring”). The hairspring is a fine spiral torsion used in mechanical wristwatches to control the rate of oscillation of the balance wheel, and thus the rate of the movement of the hands and accuracy of the timepiece. It is an integral part of the balance wheel, because it reverses the direction of the balance wheel causing it to oscillate to-and-fro (back-and-fourth). Considered the heart of a mechanical timepiece.