Contrôl Officiel Suisse des Chronométers
COSC is Contrôle Officiel Suisse des Chronomètres, the Official Swiss Chronometer Testing Institute, which is the institute responsible for certifying the accuracy and precision of wristwatches in Switzerland.
Each officially certified COSC chronometer is identified by a serial number engraved on its movement and a certification number given by the COSC.
