What does “COSC” or “superlative chronometers” mean ?

COSC, or the Contrôle Officiel Suisse des Chronomètres, is a Swiss-based institute that tests Swiss-made mechanical or quartz movements for accuracy, a mechanical chronometer must fall within a tolerance of -4/+6 seconds a day, while a quartz version is accurate to a sublime +/- 10 seconds a year. It’s the job of the COSC to ensure that the movements it receives from brands around the world meet the tolerance requirements expected of a chronometer


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