COSC

Definition

The Contrôle Officiel Suisse des Chronomètres, the Swiss institute that certifies chronometers. It tests a bare movement, not a cased watch, over fifteen days in five positions and at three temperatures, and passes it if the average daily rate stays between −4 and +6 seconds. A movement that passes may be called a chronometer, and only a small share of Swiss production is submitted. Breitling submits every movement it sells, mechanical or quartz, which is why even its SuperQuartz B71 is a chronometer. Rolex submits every movement and then tests the cased watch again to its own tighter Superlative Chronometer standard of two seconds a day. Omega's Master Chronometer begins with a COSC certificate and adds a second programme on top.