Chronometer is not a synonym for accurate watch: it is a technical-legal title obtained only after a series of standardized tests conducted by a third party or accredited laboratory. I'll say it right away because in thirty years I've seen too many buyers get confused in front of a dial where the word stands out without understanding what it really implies.
The reference standard remains COSC certification — Contrôle Officiel Suisse des Chronomètres — established in 1973. The movement is tested for 16 days in five positions and three temperatures (8°C, 23°C, 38°C). The tolerance allowed for automatic calibers is -4/+6 seconds per day. It's not negligible compared to the industrial average, but less than many imagine. Movements are tested before casing: this is an important point, because real-world conditions change performance.
In 2015 Omega, in collaboration with METAS — the Swiss Federal Institute of Metrology — introduced the Master Chronometer. Eight tests in ten days, on the fully assembled watch and not just the bare movement. Tighter tolerance: 0 to +5 seconds per day. Plus, resistance to magnetic fields up to 15,000 gauss and verification of stated water resistance. Tudor adopted the same METAS protocol for several MT56xx calibers. When I find a Master Chronometer on the bench, I expect coherence in the movement finishing too, because the test covers the entire case.
Rolex manages an in-house certification called Superlative Chronometer: movements first pass COSC, then undergo additional proprietary tests that bring tolerance to ±2 seconds per day. The standard was updated in 2026. It's not certified by a third party, but Rolex has the structure to do it in-house with comparable rigor.
There is also observatory chronometer certification — traditionally the most stringent: 15 days of testing on every single piece, protocol derived from astronomical navigation instruments. Longines uses it with Timelab for some calibers. Louis Moinet applies it individually to every watch.
Omega's Dual Metric Technology deserves a technical note: the traditional COSC method detects the seconds hand position photographically, a procedure impossible on a two-hand caliber. Omega developed an alternative system that enables COSC certification on these movements too, opening the path to Master Chronometer even for three-hand watches without the complication of a seconds hand.
From the bench: certification tells you how precise a caliber is in the lab, not how many years it will last without service. These are two different things, and confusing them is the quickest way to end up disappointed.