Frequency expresses the number of periods (cycles) that occur in a determined time. For example, the frequency of an alternating electric current, laser pulses, and commonly in watchmaking, the oscillations of the regulating organ (balance-spring). The official international unit of frequency measurement according to the International System of Units (SI) is the hertz (Hz). Hertz expresses the number of oscillations per second.

Depending on the nature of the frequency to be measured, different methods can be used to do this (electrical, electronic, acoustic, optical, comparative).

In a watch, precision is achieved by the regularity of the oscillations of its regulating organ. This means a precise and constant frequency of the balance-spring. In traditional mechanical watches, the balance-spring oscillates at frequencies between 2.5Hz and 5Hz. Thus, the balance performs up to 5 oscillations per second. While watchmakers commonly express the frequency of a regulating organ in hertz, another unit of measurement is also used. The frequency is often converted or expressed as the number of alternations (of the balance-spring) per hour A/h.

The frequency of a mechanical watch is measured acoustically using a timegrapher. The movement (or the watch) is placed on a stand where the point of contact is a microphone capturing the sound of the five beats of the escapement in each alternation. The interval between each sound is compared to a standard frequency, and a display shows the instantaneous frequency variations, demonstrating the rate variation of the watch expressed in seconds per day (s/d).