A tachymeter is a scale used in conjunction with a watch or a measuring instrument to determine speed or distance based on time. It is commonly found on the bezel of a chronograph watch, allowing users to calculate how fast an object is moving over a known distance. By timing how long it takes to travel a specific distance, the tachymeter scale provides a quick way to convert that time into speed. To use a tachymeter, you start the chronograph when the object passes a starting point and stop it when it reaches a predetermined distance, such as one kilometer or one mile. The scale on the watch then indicates the speed in units per hour. This tool is particularly useful for activities like racing or surveying, where quick calculations of speed are essential.