The frequency of a mechanical movement is the number of complete oscillations that the balance wheel performs in one second, expressed in Hertz (Hz). It's not a marketing figure: it's the parameter that determines the granularity with which the time train subdivides the second, and on which the resolution of any chronographic complication directly depends.
There are four values I encounter most often on the workbench. 2.5 Hz (18,000 vph) is the frequency of low-beat movements, rare today in series calibers but still present in certain manually-wound movements from the traditional school: the beat is slow, almost solemn, and the seconds hand advances in visible ticks. 3 Hz (21,600 vph) is an intermediate standard used by several automatic calibers from the sixties and seventies, chosen to limit wear. 4 Hz (28,800 vph) has been the dominant reference from the seventies to today: it offers a good compromise between running stability, energy consumption, and component longevity. 5 Hz (36,000 vph) is the threshold called high-beat in watchmaking: at this frequency the chronograph can measure tenths of a second, as demonstrated by the Zenith El Primero since 1969, whose caliber 3019 PHC oscillated at 36,600 vph.
The relationship between frequency and accuracy is real but non-linear. Higher frequencies statistically average the errors accumulated in each individual oscillation, but they introduce greater friction, higher energy consumption, and mechanical stresses that accelerate wear on the escapement and impulse wheel. When Rolex adopted the El Primero for the Daytona in 1988, it deliberately chose to reduce the frequency from 36,000 to 28,800 vph in caliber 4030, prioritizing robustness over long-term reliability. A precise technical choice, not a compromise downward.
On the opposite side, Longines historically developed movements up to 50 Hz for sports timekeeping, and Breguet with the Experimental Tourbillon pushed to 10 Hz to reduce the influence of shocks on oscillator regularity. In quartz, the scales change radically: a standard quartz oscillates at 32,768 Hz, the Bulova Precisionist reaches 262,144 Hz, with the seconds hand flowing continuously instead of advancing in ticks.
The unit of measurement alternative to Hz is vph, vibrations per hour. The conversion is direct: simply multiply the Hz by 3,600. A figure worth always checking in the technical specifications, because some datasheets confuse simple alternations with complete oscillations, doubling the nominal value.