Controle Officiel Suisse des
Chronometeres (The Swiss Official Chronometer Control). COSC is an independent association governed by the Swiss Civil Code.
Watch companies desiring the chronometer designation on their best pieces send
movements to COSC. (COSC tests uncased movements; the companies case the
movements after the tests.) COSC issues a performance certificate for each
which successfully passes the tests.
Rolex has more watches tested every year than any other.