The balance wheel is the regulatory heart of every mechanical movement: it oscillates back and forth at constant frequency, and each oscillation authorizes the escapement to release a tooth of the escape wheel. Without it, there is no measurement of time. It always works in tandem with its hairspring, which recalls it to center after every excursion.
The geometry of the support matters as much as the wheel itself. The balance cock — the cantilevered bridge that holds the upper axis — is the historical solution, often hand-engraved as in the Kudoke 3 Nocturne, where it bears the symbol of manufacturing infinity. The balance bridge, instead, traverses the movement on two fixing points: Rolex's 3135 caliber adopts it precisely for this reason, achieving better shock resistance and better coherence in six positions. Panerai takes the concept even further in the P.980, with a traversing balance bridge that directly contributes to six-position certification.
Frequency regulation is achieved by modifying the inertia of the wheel. In the classical system, you act on a regulating spring that touches the hairspring: it works, but introduces sensitivity to magnetic and thermal variations. The free-sprung balance eliminates that contact: the hairspring is free, and frequency is corrected by shifting eccentric screws on the wheel. In the KV21 these masses are in rose gold; in the Carl F. Bucherer Balancier 3 they are six gold screws; in the Seiko Astron SA-30W they are four cylindrical blocks. The practical result is superior stability over time, less drift after accidental magnetization, less variation when changing position. This is why free-sprung has become the standard of serious haute horlogerie.
The hairspring is the other half of the pairing. In traditional steel it reacts to magnetic fields and temperature variations. Industrially consolidated alternatives are Nivachron — a titanium and niobium-based alloy used by ETA and Sellita among others — and Rolex's Parachrom, a proprietary alloy. Both significantly reduce magnetic drift. Flat silicon, used by Breguet in the 581DPE caliber, goes further: it is non-magnetic, requires no lubrication, but machining is delicate and costs remain high. The cylindrical hairspring with Berthoud terminal curves of the Ferdinand Berthoud FB 3SPC represents instead the opposite solution: hand-crafted, 18th-century technique, demonstrable metrological results.
Finally, there are double balance wheel architectures. AP introduced the double balance wheel in 2016 in the Royal Oak Openworked caliber 3132. Armin Strom instead uses resonance: two independent balance wheels that influence each other mutually tend to synchronize and compensate for their respective deviations. From the workbench I have seen movements with this configuration: the chronometric effect exists, although its magnitude depends greatly on assembly precision.