The sub-dials on a chronograph watch vary in number and function depending on the chronograph's design and intended use. When the chronograph button is pushed to activate the stopwatch, the sub-dials start tracking the elapsed time.
Generally, a chronograph has one sub-dial that records elapsed minutes and is graded in segments of 30, while another sub-dial graded in segments of 12 records elapsed hours. The third sub-dial may show the elapsed seconds, or, if the seconds are counted by a central sweeping hand on the main dial, this sub-dial may count fractions of a second. If the third sub-dial runs continuously even when the stopwatch is not running, it is most likely used in conjunction with the main dial and is independent of the chronograph feature.