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ISO standard

When it comes to precision, COSC refers, for mechanical watches, to the international ISO 3159 standard and, for quartz watches and marine chronometers, to internal directives based on that standard. Standard ISO 3159 has been enshrined in Swiss law.

This standard indicates in particular an obligation to satisfy seven criteria associated with rate accuracy: the average daily rate, average rate variation, greatest rate variation, difference between the horizontal and vertical positions, biggest rate difference, thermal variation and rate-resumption. The minimum requirements are defined in seconds per day.