A chronograph watch has two independent functions. One is to provide the time of day and the other is to measure intervals of time.
The first commercialized chronograph was invented 1821 and originally patented in 1822 by Nicolas Mathieu Rieussec for King Louis XVIII who hired the inventor to create a device that could measure the exact duration of each horse race.
The word Chronograph comes from the Greek words for time (chronos) and writing (graph). Earlier forms of the chronograph date back to 1776 and actually used "writing" on the dial with a pen attached to the index. The length of the pen mark would show the lapsed time.
Today, the method is much simpler. The stopwatch function is electronic or digital and works by pushing a button, usually located at the 2 o'clock position, to start and stop the recording. Although there are more sub-dials in more complicated timepieces, the standard chronograph watch has three sub-dials. The hand of smallest one revolves once every second, the next one revolves once every sixty seconds, and the third one revolves once every 60 minutes. The three dials interact with one another to record the lapse time between start and stop.