The hairspring is the component that determines the frequency of the oscillator and, consequently, the precision of a mechanical watch. It is a metal ribbon wound in a flat spiral (or, in certain cases, cylindrical) that provides the restoring force to the balance wheel: whenever the balance wheel deviates from center, the hairspring brings it back with a force proportional to the displacement. This behavior, defined by the harmonic oscillator equation, is what makes it possible to measure time.
The geometry of the termination is decisive. A flat spiral without corrections expands and contracts asymmetrically, shifting its center of gravity with each oscillation and introducing rate errors. To compensate for this defect, Abraham-Louis Breguet developed at the end of the eighteenth century what we today call the Breguet overcoil: the last coil is bent upward and inward, so the hairspring breathes more concentrically. Even today it is one of the most effective adjustments a regulator can make to a quality caliber. The Phillips (external) and Grossmann (internal) terminal curves work on the same principle: they modify the shape of the terminal coils to improve isochronism. Having both on a single hairspring requires manual bending and is rare even among manufacturers claiming haute horlogerie craftsmanship.
The cylindrical hairspring, inspired by eighteenth-century marine chronometers, is a radically different geometry: the coils develop on a cylinder rather than on a plane. It breathes concentrically, improves the regularity of oscillation, and reduces positional sensitivity. Ferdinand Berthoud and H. Moser & Cie. produce it by hand today at Precision Engineering AG. It is costly, bulky, and extremely difficult to regulate, which is why it remains an absolute rarity even in haute horlogerie.
Material is the other major discriminant. Beryllium steel was the standard for decades; then proprietary alloys arrived. Rolex produces the Parachrom in a paramagnetic niobium-zirconium alloy — that characteristic blue coloring is not aesthetic but a consequence of the alloy itself — and the Syloxi in silicon, used on compact calibers like the 2232. Swatch Group developed Nivachron, a titanium-niobium alloy resistant to magnetic fields and thermally stable, used on the Powermatic 80 and Rado movements. Silicon itself guarantees lightness and absence of magnetization, but it is fragile: any direct impact can shatter the hairspring. Chopard developed an antimagnetic solution without silicon for the Alpine Eagle 41 AM, resistant up to 2,000 gauss.
Only very few manufacturers produce their own hairsprings internally: Minerva at Villeret, Schwarz-Etienne, and the manufactories of the MELB Luxe group through Precision Engineering AG. All others purchase from external specialists, primarily Nivarox-FAR. When a brand claims internal production of the hairspring, it is worth asking exactly who owns the winding and heat treatment machinery.