10.7mm thick, with a stopwatch inside.

A tachymeter is a speed scale. Start the chronograph as you pass one mile marker, stop it at the next, and the number the seconds hand points to on the outer scale is your average speed in miles per hour. It works for kilometres too, and for anything else you can time over a known distance.

It is also why chronographs look the way they do. The scale gives the dial its ring of numbers, and the three registers give it its face. On the AT2387 those registers count running seconds, chronograph minutes and a 12/24-hour display.

Citizen’s US site no longer lists the AT2387-52E, which usually means a reference has moved out of the current catalogue while stock continues to sell through retailers. The figures on this page are cross-checked across those retailer listings rather than a manufacturer page.

What they agree on: 43mm across, 10.7mm thick, a mineral crystal, a steel bracelet with adjustable links, and 100m of water resistance. That is enough for swimming but it is not a dive watch. Nothing here is measured or timed by us.