The word «chronometer» is not a synonym for a precise watch: it is a technical title earned by passing tests certified by a third-party body. In modern watchmaking, the baseline reference is the COSC, Contrôle Officiel Suisse des Chronomètres, which accepts automatic movements with a deviation between −4 and +6 seconds per day, measured in the laboratory over 16 consecutive days, in five positions and at three different temperatures. Passing that test is not trivial, but it is also the lowest rung on the scale.
The ISO 3159 standard codifies the same criteria at the international level. Longines, for example, has its L836.6 caliber certified by TimeLab Geneva according to this standard after casing assembly, not before: a choice that guarantees the movement meets tolerances in the finished watch, not on the regulation bench. This is a distinction worth understanding. Traditional COSC certifies the raw movement; certifying after casing is more stringent because the case introduces real-world variables of use.
Rolex takes a further step with Superlative Chronometer Certification, which tightens the tolerance to −2/+2 seconds per day: an internal standard stricter than COSC, applied to the complete watch. Omega instead relies on METAS, the Swiss Federal Institute of Metrology, which brings tolerances to 0/+5 seconds per day, adds resistance tests to 15,000 Gauss and verifies water resistance and declared power reserve. For two-hand movements without central seconds, Omega developed Dual Metric Technology, which allows Master Chronometer certification even without the seconds hand that the traditional photographic method requires.
Regulation in five positions is the manual work that precedes any certification: the movement is timed with the case in the 12, 3, 6, 9 o'clock and horizontal orientations. The more positions that are balanced, the more stable precision is in real-world use. Some manufacturers speak of «near-COSC accuracy» for uncertified movements but regulated with similar care: this is a commercial claim, not a technical title.
The term has roots in the history of navigation. Marine chronometers — deck-watches included — often used the detent escapement, such as John Arnold's spring detent, chosen for minimum friction and great stability in sea conditions. Urban Jürgensen learned that technique directly from Arnold in London. Today the detent escapement survives in pieces like Bernhard Lederer's Central Impulse Chronometer, with dual remontoir d'égalité providing constant impulse to the anchor. The principle is the same as it was then: to isolate the balance wheel from variations in spring force.
A terminological note I often see ignored: «chronometer» and «chronograph» are not the same thing. The chronometer is a precision certification. The chronograph is a complication that measures time intervals. A watch can be both, one of the two, or neither.