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A stopwatch is a handheld timepiece designed to measure the amount of time elapsed from a particular time when it is activated to the time when the piece is deactivated.

A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stopclock. In manual timing, the clock is started and stopped by

a person pressing a button. In fully automatic time, both starting and stopping are triggered automatically, by sensors.

A typical mechanical analog stopwatch.The timing functions are traditionally controlled by two buttons on the case. Pressing the top button starts the timer running, and pressing

the button a second time stops it, leaving the elapsed time displayed. A press of the second button then resets the stopwatch to zero. The second button is also used to record

split times or lap times. When the split time button is pressed while the watch is running, the display freezes, allowing the elapsed time to that point to be read, but the watch

mechanism continues running to record total elapsed time. Pressing the split button a second time allows the watch to resume display of total time.

Mechanical stopwatches are powered by a mainspring, which must be periodically wound up by turning the knurled knob at the top of the watch.