For a watch to earn the title of "chronometer", the movement has to pass a very strict battery of test under very strict control at the most renowned official testing organization, the C.O.S.C (Controle Officiel Suisse des Chronometres). The testing last for 15 consecutive days. For the first ten days, the movements are tested in five positions at 20 degrees Centigrade. Three of the positions are vertical: 3 o'clock left, 3 o'clock up, and 3 o'clock down. Two test are horizontal: dial up and dial down.
After that, the movement is left in the horizontal, dial up position, and the temperature is lowered to 4 degrees Celsius for one day. Then it is raised back to 20 degrees Celsius for one day. Then the temperature is increased to 36 degrees Celsius. For the last two days of the test, the movement is moved back to vertical 3 o'clock left position and the temperature is lowered back to 20 degrees Celsius.
- The timekeeping of the movement is recorded daily, and at the end of the 15 day test, calculations are made in six different areas. The different areas are:
- 1. mean daily rate in the different positions.
- 2. mean variation.
- 3. maximum variation.
- 4. greatest difference between the mean daily rate and any individual rate.
- 5. variation of rate per 1 degree Celsius.
- 6. resumption of rate.
- The variances allowed are very small. The mean daily rate must be within -4 and +6 seconds per 24 hours.