Horology glossaryDefined term / cosc
COSC
Contrôle Officiel Suisse des Chronomètres, the official Swiss agency that tests and certifies individual movements as chronometers before casing.
Authentication note
Why this detail matters.
COSC certifies movements, not finished watches, and issues a certificate per individual movement serial number, which some brands preserve with the watch. A dial marked "Chronometer" should correspond to a movement built to COSC's tolerances, so the absence of expected chronometer markings on a model that should carry them is worth investigating.