The balance-spring is a small spring which through its elasticity regulates the oscillations of the balance. It is attached at its inner extremity to the axis of the balance and at its outer extremity to the cock. The flat balance spring, invented by the Dutch mathematician Huygens in 1675, had established a degree of isochronism which still left something to be desired. The flat balance-spring was made of copper or iron and had only a few coils. Though imperfect, it gave the balance what it needed to become as accurate as the pendulum of a clock.
In 1795 Abraham-Louis Breguet solved the problem by upraising the spring’s last coil and reducing its curvature, to ensure the concentric development of the balance-spring.
Thus endowed with the "Breguet overcoil", the balance spring henceforth became concentric in form. Watches gained in precision, and the balance-staff erode less quickly. 1880-1910, many manufacturers inscribed the words "Spiral Breguet" in large letters on their watches.