C.O.S.C., or Contrôle Officiel Suisse des Chronomètres, is an organization that certifies the accuracy and precision of watches made in Switzerland. It is a highly sought after certification, and any watch that has passed its rigorous tests is considered to be of the highest quality. C.O.S.C. tests the accuracy of a watch over 15 days and in five different positions to determine its accuracy. To earn a C.O.S.C. certification, a watch must have an average daily accuracy of -4/+6 seconds. Receiving a C.O.S.C. certification is a sign of the highest craftsmanship and quality of the watch, and is highly prized by watch collectors.
