The principle of operation of all mechanical clocks rests on the combination of the following three functions:

• A source of energy that can produce rotary motion (here, a driving weight)
• A regulator:  A pendulum measures time precisely and without variations. The escapement system, linked to the pendulum,  enables control of the rate of energy release.
• A display:  graduations and needles ("hands") provide access to the measured information.

Escapements generally use an anchor, as shown in this animation. For small angles (<5°) of oscillation of the pendulum, one approaches the condition of isochronism:  the pendulum's period depends practically exclusively on local gravitational conditions and the length of the pendulum (bit not on either the mass of the pendulum or the amplitudes of its oscillations).  For example, in Paris, a meter long pendulum has a period of two seconds. Lengthening the pendulum increases its period of oscillation.