A chronograph is a function for measuring elapsed time, added to telling hours and minutes. A hand, usually started and stopped by pushers, sweeps a counter. Subdials record longer intervals. On a used piece, the useful checks are simple: the pushers move and return, the hand starts and stops, and the counters reset together.
A tachymeter scale is often printed on the bezel so elapsed seconds can be read as a rate. That scale is defined in the glossary. This note is about the function. The model that carries it is a reference page, found from Families.
The complications index explains how these notes are organized. Date and GMT are separate notes. Case size still belongs on Fit.