The Breguet Overcoil is a type of hairspring design that features an upwardly curved outer coil, improving the timekeeping accuracy and isochronism of a watch.
The Breguet overcoil (also called Breguet spring) is a hairspring design where the outermost coil curves upward and inward, creating a terminal curve that allows the spring to expand and contract more concentrically. Invented by Abraham-Louis Breguet, this geometry improves isochronism — consistent timekeeping regardless of the mainspring's power level — making it superior to flat hairsprings.
Standard flat hairsprings expand asymmetrically because the outer coil's end attachment disrupts concentric breathing. The overcoil's raised terminal curve allows the spring to breathe more evenly, maintaining a consistent center of gravity and reducing positional errors. This improves isochronism — the balance wheel oscillates at a more constant rate regardless of remaining mainspring power.
Breguet overcoils are found in high-end mechanical watches where precision is paramount. A. Lange & Söhne and Patek Philippe use overcoils in their most prestigious calibers. Breguet (the brand) naturally features them across its lineup. The overcoil requires more skill to produce and regulate than a flat spring, so its presence indicates superior movement quality and craftsmanship.