Mileage indicators were used to determine distances between large cities. They helped with travel planning and indirectly also with the calculation of postage costs. The desired distance can be read at the intersection of 2 coordinates. As a precaution, the mileage indicator shown here contains a detailed set of instructions. The decorative engraving on the copper plate refers to the postal system and land survey. The winged messenger Mercury hands a letter to a city goddess.