You simply place it at the starting point (map with scale 1:50 000) and move it until your will reach your final destination. After that you can start to read from the device. Each segment represents 1 km. On the other side of the item, there is a measuring scale in cm. Each segment represents 1 cm and 1 cm on map with scale 1:50 000 = 500m = 0,5 km. Item is very small and light, so you can easily wear it in your pocket.