Also spelled chronometre, this Swiss term that defines the performance of a mechanical watch. Most commonly, the COSC chronometer certification requires accuracy of
-5 to +6 seconds per day.
It's also known as ISO 3156. See also ISO 3158 for the positions used.
Chronometer is also the highest ETA movement grade, referring to watches that have their best materials and a COSC certificate.
- Rolex claims 'superlative chronometer'; this is meaningless marketroid-speak. It's the same spec.
- The Grand Seiko and Patek Seal, and JLC 1000 hours specs are all more demanding; the COSC spec has been fixed since the late 1960's and really should be made more demanding.
- Watchbore article on COSC - start here
- COSC versus Grand Seiko thread
- Harry Bishop has a nice post on COSC's exclusion of Seiko in their competitions.