The balance and spring assembly represents the heart of the watch; it regulates the flow of time by means of the oscillations of the balance and is responsible for the accuracy of the watch.

It is composed of a statically poised two or three-arm fly wheel (the balance - 1) as well as a spring (a steel spring thinner than a hair's breadth - 2).

The balance performs a circular to-and-fro motion and divided up time into equal units.

1. The pallet-lever gives an impulse to the balance to make it perform one rotation: the TICK sound
2. During this rotation, the balance-spring is compressed, thereby balancing the energy distributed

3. Then the balance-spring uncoils and forces the balance to swing back : the TOCK sound

4. The to-and-fro principle is achieved, corresponding to an oscillation

5. A new impulse is then given by the pallet-lever

An oscillation is composed of 2 vibrations.

Frequency is the number of oscillations in one second. This term is often used to describe a watch calibre and is expressed in vibrations per hour:

• A frequency of 2.5 Hertz is equivalent to 18,000 vibrations per hour (meaning 5 vibrations per second)
• A frequency of 3 Hertz is equivalent to 21,600 vibrations per hour

• A frequency of 4 Hertz is equivalent to 28,800 vibrations per hour

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  A frequency of 5 Hertz is equivalent to 36,000 vibrations per hour (meaning 10 vibrations per second)

The energy thus regulated is transmitted to the hands by a set of wheels.