On the dial or bezel of a watch, a scale is a graduation that transforms a time measurement into different information: speed, distance, heart rate, depth, power reserve. The chronograph measures the interval; the scale interprets it. Without this distinction, we confuse the instrument with the reading.
The tachymetric scale is the one I encounter most often at the counter. I start the chronograph at the first kilometer, stop it at the second: the hand directly indicates average speed in km/h. The scale is built on the formula 3600 divided by time in seconds, which explains why the numbers become denser toward higher values — compressing the same speed variation always requires less arc. It can be printed on the dial, engraved on a fixed bezel, or, as on the Montblanc 1858, on a 1000-unit rotating disc. The TAG Heuer Carrera Split-Seconds engraves it on a curved anthracite bezel shaped to follow the crystal and reduce parallax error: a technical detail, not decorative.
The telemeter scale operates on a different physical principle: the delay between visual event and sound. I start the chronograph at the flash, stop it at the thunder, read the distance. It was a military instrument in theaters of war to calculate range. Today I find it almost only on vintage chronographs or historical reinterpretations like the Eberhard Chronographe 1887.
The pulsometer scale is another medical legacy: by counting heartbeats for 15 or 30 seconds with the chronograph running, the hand directly indicates frequency. The Chrono-Felix Medicus brought it to the center of the dial with a snailed finish. It still works, if you know how to use it.
There are also lesser-known but equally precise scales. The double scale of the Doxa Sub 300 — depth in feet on the outer ring of the bezel, immersion time on the inner one — is a complete underwater navigation instrument in itself. The linear power reserve scale of the Hublot MP-17 uses a rack mechanism to scroll through 240 hours of autonomy: no rotating index, everything linear and readable. The Zenith Chronomaster Sport instead uses a 0 to 10 scale for tenths of a second, exploiting the 5 Hz frequency of the movement.
The railway-track minutes scale on the A. Lange & Söhne 1815 dials is a special case: a minutes scale shaped like railroad tracks on the periphery, copied directly from F.A. Lange pocket watches from the 1870s. It doesn't measure anything special — it measures minutes — but it does so with a visual hierarchy I haven't seen matched in many other German dials.