Chronometer term is used to denote a very precise watch. These watches usually have a deviation of less than 5 seconds per day. Only watches that pass the Official Swiss Testing Office for Chronometers (COSC) test standards may use the chronometer word. The test consists of having the watch in observation for a few days in several positions and different temperatures to ensure the watch will keep precise time under extremes conditions. Watches that are Chronometer certified usually come with the COSC certificate. For many years Rolex has been the largest producer of Chronometer certified watches.

Official Swiss Testing Office Chronometer Certificate

Rolex Submariner with Superlative Chronometer Officially Certified written on the dial

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