A tachymeter is a feature commonly found on chronograph watches that enables the measurement of speed over a known distance. The tachymeter scale, usually inscribed on the watch bezel or dial, allows the user to determine speed based on the time taken to travel a set distance. By utilizing the chronograph function to measure elapsed time, the tachymeter provides a convenient and practical tool for those involved in motorsports, aviation, or other activities where speed calculations are essential.