What COSC tests
The Contrôle Officiel Suisse des Chronomètres (Swiss Official Chronometer Testing Bureau) was founded in 1973 to consolidate the older cantonal observatory trials into a single industry-scale certification body. COSC tests untimed mechanical movements (not finished watches) over 15 consecutive days at three temperatures (8°C, 23°C, 38°C) and in five positions (crown left, crown up, crown down, dial up, dial down). Seven criteria are scored, but the most important is mean daily rate: -4 to +6 seconds per day for movements 20mm or larger.
What it doesn't test
COSC tests movements only, not the cased watch. After certification, the movement is shipped to the brand, cased up with hands and dial, and sent to retail. The casing process can shift the rate slightly from the COSC-certified spec. To address this, several brands run watch-level certification on top of COSC: Rolex Superlative Chronometer (-2/+2 in case), Omega Master Chronometer / METAS (0/+5 plus 15,000-gauss anti-magnetism), Breitling chronometer-certification, Patek Philippe Seal (Patek's in-house alternative since 2009).
Who submits to COSC
Rolex alone submits roughly 1 million movements per year; every modern Rolex carries the dial mark "Superlative Chronometer Officially Certified", which builds on COSC plus their own in-house verification. Omega certifies the great majority of its mechanical watches as a precondition for the in-house Master Chronometer programme. Breitling certifies its entire mechanical catalogue. Tudor, Mido, Chopard, and a handful of independents contribute the rest. Total annual output: ~2 million COSC certifications.
What a COSC watch does for you
A COSC watch will run within -4/+6 sec/day for the first few years of ownership, drifting wider as oils dry and parts wear. After 5-7 years a service brings it back to spec. The certification is per-movement, not per-brand: every individual COSC-certified movement has been tested. This is different from "chronometer-grade" claims (no certification, just a marketing claim) and from older "in-house regulated to chronometer spec" (regulated but not individually tested).
What COSC is not
COSC is not the highest standard in modern watchmaking; it is the industry baseline for "chronometer". Master Chronometer (Omega + METAS) is stricter (0/+5 in case + 15,000G anti-magnetism + multiple-position case-level testing). The Geneva Seal (Poinçon de Genève) tests both finishing AND timekeeping. The Patek Seal (Patek's 2009 internal cert) is rate -3/+2 for cased watches. None of these replace COSC for the volume tier; they layer on top.
Why it matters
For a buyer, COSC certification is a useful but minimal indicator. It guarantees the movement was individually tested, not regulated to a brand average. It is not a guarantee of better build quality, finishing, longevity, or anti-magnetism. A COSC-certified movement housed in a poorly-finished case is still a poorly-finished watch. The certification is a baseline credential, useful when comparing entry-tier mechanicals; meaningless when comparing CHF 50,000+ haute-horlogerie pieces (which are typically held to stricter internal standards than COSC).