What does “COSC” mean?

COSC is the Contrôle Officiel Suisse des Chronomètres, the Swiss official chronometer testing institute. A watch described as a chronometer has a movement that passed its tests, which measure accuracy over about two weeks in several positions and at different temperatures. For a standard mechanical movement, the permitted average daily rate is between minus 4 and plus 6 seconds.

In practice, COSC certification tells a buyer that the movement met an independent accuracy standard before it was cased, which is why it often appears on dials as “Superlative Chronometer” or “Chronometer Officially Certified”. Some brands add their own stricter in-house tests after casing, such as Rolex's own standard or Omega's Master Chronometer testing with METAS. COSC is not a guarantee of how a watch runs years later, since service history and wear matter more, so dealers still timegraph watches before selling.

A buyer might ask: “Is this the COSC version or the standard movement?” For some models the chronometer version is a separate reference and attracts more interest.