The COSC — Contrôle Officiel Suisse des Chronomètres — is the independent Swiss body founded in 1973 that issues chronometer certificates for watch movements. It does not certify watches: it certifies movements. The distinction is important, and I'll return to it in a moment.
The testing protocol lasts sixteen consecutive days. The movement is measured in five positions and at three different temperatures. To obtain the certificate, the average daily rate must fall between −4 and +6 seconds per day. That asymmetrical tolerance is not random: a watch that gains time depletes its power reserve faster, and the standard treats it differently from one that loses time.
In my experience at the bench, the COSC certificate gives real assurance about the behavior of a raw movement in the laboratory. It does not automatically guarantee the same result once the movement is cased, regulated on the wrist, and worn daily. Variables such as body temperature, the intensity of the wearer's movements, and bearing wear all affect actual performance. A COSC-certified movement that runs at +8 seconds per day on the customer's wrist is not a defective movement: it is simply one that needs adjustment.
COSC certification is the starting point for stricter standards. Rolex certifies its movements COSC, then subjects them to a second internal check called Superlative Chronometer, with tolerance reduced to ±2 seconds per day on the assembled watch. METAS — the Swiss Federal Institute of Metrology — does something structurally different: it tests the complete watch (not just the movement), imposes tolerances of 0/+5 seconds per day, and adds magnetic resistance testing up to 15,000 gauss and verification of declared water resistance. METAS certification requires COSC as a prerequisite, but surpasses it on all parameters.
The expression near-COSC accuracy found in certain press releases indicates a caliber that approaches −4/+6 without officially passing the test: it is marketing, not metrology. It is worth knowing.
Finally, an error I often see in technical datasheets: Master Chronometer is not a synonym for COSC. It is the commercial name Omega uses for its own METAS certification. Different brands, different bodies, different protocols.