What exactly is a chronometer ?  A chronometer may thus be defined as follows:

The term chronometer is often wrongly applied to timekeeping instruments fitted with an additional mechanism that may be set in motion by pushbuttons to enable measurement of the duration of an event.

Such an instrument is in fact a chronograph or chronoscope. It may of course be chronometer-certified, provided it meets the criteria set for the standard.

In its technical regulations, the COSC has included an additional condition, namely the permanent display of the seconds.

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A chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions and at different temperatures, by an official neutral body (COSC).

Each chronometer is unique, identified by a number engraved on its movement and a certification number given by the COSC.

Each movement is individually tested for several consecutive days, in 5 positions and at 3 temperatures.

Each movement is individually measured. Any watch with the denomination "chronometer" is provided with a certified movement.