A watch that has been independently tested by the Official Swiss Chronometer Testing Institute (COSC) in Switzerland (or any other official governing body in other countries). In the case of the former, watches are tested over the course several days in six different positions at three different temperatures, while remaining accurate to within -4/+6 seconds per day for mechanical watches and ±0.07 per day for quartz watches. (Learn more about chronometers here.)
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