COSC

Contrôle Officiel Suisse des Chronomètres (C.O.S.C)

Watch Mechanics Not to be confused with the word 'Chronograph'…

A chronometer is a watch that has passed strict tests for accuracy in an observatory, or testing bureau, of the Swiss official chronometer testing organisation and has been issued with an official rating certificate by the " Controle Officiel Suisse des chronometres " (C.O.S.C.)

It's tested for 15 days and 15 nights under a variety of different conditions designed to mimic the effects of the watch being worn at 5 different 'wearing' positions at 3 temperatures: cold, normal & warm.

To pass and gain a Chronometer certification, the watch must maintain an accuracy of between minus 4 and plus 6 seconds per day.

Reasonable accuracy expectations by movement type:
Modern mechanical watch, Non-Certified +/- 10 seconds a day
Modern mechanical watch, Chronometer Certified -4/+6 seconds a day
Modern quartz watch +/- 15 seconds a month