Ferdinand Berthoud FB 2RE.2 : Superlative Chronometric Performance Verified by Two Independent Testing Laboratories to 0.5 Seconds/Day!
For a watch to qualify as a COSC certified chronometer, the movement not the cased watch) has to be precise to -4 sec to +6 sec per day and that’s more than precise enough for most brands. But Ferdinand Berthoud is not most brands.