Home Continuing Series Historical Horology Historical Horology: What Is A Chronometer?

Historical Horology: What Is A Chronometer?




While today’s post isn’t necessarily a history lesson in and of itself, it is concerning a certification we’ll often see applied to higher-end mechanical movements – COSC.

Often times, it can be easy to confuse a chronograph and chronometer, as they are spelled rather similarly.  However, a chronometer-certified movement is a movement that’s held to a higher standard of testing and accuracy – and it doesn’t necessarily have to have chronograph functions (though it may).


Most of the times that we see COSC, it’s truly from the Swiss Authority (their page is here; it should be noted that there are other in-house and country-specific testing authorities), and they do a variety of tests which conform to a specific ISO Standard (number 3159), which includes a variety of tests, such as:

  • Testing over a span of 15 days
  • Testing, within that span, in five different positions over several different temperatures

Once those tests have been done, the movement must maintain accuracy to within -4/+6 seconds per day.  In short, it’s going to be the most accurate movement you can get, that should handle most any situation you’re likely to find your watch in.

How would you know your watch truly has a COSC-certified movement?  First and foremost, you should have COSC-certification paperwork, which will include the test results.  Second, if you actually took the watch apart, you’ll find a COSC serial number has been engraved onto the movement itself.


Given that only about 3% of Swiss watches made today are comprised of these chronometer movements, it’s not surprising that they’ll command a premium over a movement with similar (or identical) specs that can’t meet the stringent test criteria.

If you’d like to read more, head on over to this article at A Timely Perspective (link).

All pictures courtesy of A Timely Perspective


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