A chronograph measures a duration: it has a hand you can start, stop and reset. A chronometer is a watch whose accuracy has been certified to official standards. One watch can be both, as the Fondation Haute Horlogerie points out: a chronograph can be certified as a chronometer, and not every chronometer is a chronograph.
The chronograph is a function
The word names a mechanism added to the watch. Its pushers, on the side of the case, control an extra hand and counters. It is easy to spot: two pushers around the crown, small subdials for the minutes and sometimes the hours measured. Our article how a chronograph works opens the case and shows the column wheel, the clutch and the reset hearts.
The chronometer is a certificate
In Switzerland the title is awarded by the Swiss Official Chronometer Testing Institute. COSC explains that certification is granted by an official, neutral authority under the ISO 3159 standard. Each certified chronometer bears a unique number engraved on its movement and comes with a certificate. Many brands also print "Chronometer Certified" on the watch.
What happens in the laboratory
COSC describes tests lasting 12 to 20 days depending on the type of movement. Movements first rest for 12 hours in an enclosure at 23 °C. Several days of measurements follow in 24-hour cycles, with the position changed daily: 6 o'clock, 3 o'clock, 9 o'clock, dial up, then dial down. Finally, dial up, the movements go through chambers at 8 °C, 23 °C and 38 °C, 24 hours each.
This protocol is what gives the word its value. A wristwatch changes position and temperature all day long; a chronometer has shown that it keeps its rate in those conditions. For mechanical watches, our article on mechanical and automatic movements gives the −4 to +6 seconds per day tolerance set by ISO 3159.
What COSC certifies
COSC mainly certifies mechanical wristwatches, but also pocket watches, fixed timepieces and quartz wristwatches. Wristwatch movements with a balance-spring oscillator follow ISO 3159; other categories, quartz included, follow in-house specifications inspired by that standard (COSC).
Reading a dial correctly
Two pushers and counters: that is a chronograph. "Chronometer" on the dial signals a certification; for a Swiss watch, the certificate and the number engraved on the movement link it to COSC. Both clues can appear on the same watch.
Frequently asked questions
Where does the word chronograph come from?
From the Greek for "time writing". The Fondation Haute Horlogerie relates that Rieussec's device of 1821 left a drop of ink on a rotating dial.
Can a quartz watch be a chronometer?
Yes: COSC also certifies quartz wristwatches, under adapted specifications.
Further reading
The School's Absolute chronometry module explains what makes a movement accurate, and the Chronograph module details the mechanics of measurement. To regulate your own watch, read our guide to adjusting accuracy, then discover the Décalibre School.