A chronograph is a watch that features a stopwatch function. A tachymeter is a scale on the watch bezel that allows you to measure speed over a given distance. Chronographs are often used by sportspeople and other professionals who need to time specific events. They can also be used to track lap times in distance races such as marathons or triathlons. Meanwhile, tachymeters can be used by anyone who needs to measure speed or distance. They generally feature a rotating bezel that can be turned to provide more accurate measurements.