The word "chronograph" is a combination of the Greek words for "time" and "writing". The word is said to refer to the earliest from of chronograph, a horizontal clock equipped with a pen-like device and a small reservoir of ink. When the user activated the device, it left an ink mark on the clock dial showing how much time had elapsed. In modern chronograph watches, the exact method of operation varies from model to model. There are a few general rules, however. In every chronograph, the stopwatch function operates independently of the timekeeping function, so while you are timing a race or a lap in the pool, your watch will keep track of the time of day.
To start and stop the chronograph function, the wearer pushes a button. Some chronographs are started and stopped with the same button, others have separate start and stop button. Depending on the style of the watch, the chronograph will keep track of the seconds either on the watch's large main dial, with a center seconds hand, or in a smaller subdial. Digital models showed the elapsed time on the LDC display. After the wearer stops the chronograph and notes the elapsed time, he returns the chronograph and notes the elapsed time, he returns the chronograph hand to its original position by pressing another button.
Most chronographs are able to keep track not only of the seconds, but also the minutes that have elapsed since the chronograph began running. Some even keep track of the elapsed hours. Some chronograph watches have yet another subdial which times fractions of a second, or a scale at the edge of the dial which allows the wearer to read fractions of a second. Typically digital chronographs give readings to the 1/100 of a second; analog quartz chronographs give reading to 1/10 of a second; mechanical chronographs give readings to a 1/5 of a second.
The hands on all chronographs subdials return to the their starting position simulteneously with the seconds hand when the wearer pushes the appropriate button.
Finally, don't confuse a chronograph with a chronometer. A chronometer is an extremely accurate timepiece that has passed certain timekeeping tests adminstered by an official testing agency in Switzerland or France. A chronometer may or may not be a chronograph.