A high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions and at different temperatures, by an official neutral body COSC. Each chronometer is unique, identified by a number engraved on its movement and a certification number given by the COSC after it has been individually tested uncased for several consecutive days, in 5 positions and at 3 temperatures and it runs at a rate of -4/+6 per day or better.

Rolex “Superlative Chronometers” are tested to a rate of -2/+2.