Though the Swiss set the standard, the English coined the word. In the quest to calculate the longitude, the Englishman Jeremy Thacker, a clockmaker of extraordinary skill, created a series of increasingly accurate clocks, well beyond what had had previously been achieved. The clocks were intended to be used aboard ships and had to keep absolutely accurate time in order to allow for calculating longitude precisely, and thus fixing the position of the ship in the vast ocean. The last of Thacker's chronometers was actually more like a very large pocket watch.
Other clockmakers - and increasingly watchmakers - copied from and improved upon Thacker's final chronometer, producing watches of greater and greater precision. The original standards for accuracy were actually far more exacting than those currently maintained by the COSC in Switzerland, so very few watches were certified as chronometers and thus very few were actually produced and sold. The original chronometer requirements for mechanical watches are no longer necessary, since digital watches can already exceed them, but chronometers are still manufactured and sold (and resold) today as examples of extremely fine precision engineering. The use of jewels and precious metals in watches owes its origins to chronometers - these materials were used to make the mechanical movements as precise as possible.
Officially Certified Chronometers
Chronometers are no longer rare. More than a million Officially Certified Chronometer certificates are issued each year. Those watches that pass the most rigorous of the COSG's tests are rewarded with a special serial number and, indeed, the right to call themselves "chronometers" at all. Chronometers must meet several requirements: they must display the seconds; they must be highly precise; they must demonstrate that precision over the course of several consecutive days of testing; they must perform precisely in different positions and at different temperatures.
Chronometers in Everyday Use
This may sound extreme, but such reliability was crucial for individuals and groups that had to rely on the precision of portable mechanical timekeepers in the past. Pilots watches, for instance, needed to perform as chronometers (and those that did not had to be hackable). The famous B-Uhren, commissioned by the German Luftwaffe and manufactured by several companies, were chronometer-certified pilots watches - or chronometers that were used as pilots watches, depending on your perspective. Interestingly, despite the many years between Thacker's invention and the production of the B-Uhren, chronometers were still tightly bound to use in navigation.
First at sea, then in the air. And, still today, at sea again - or at least under it. Some diving watches, like the Rolex submariner, are also certified chronometers. Divers must be able to reliably track how long they've been under and how much time they have to surface.