A scale bar is a graphic that shows

A scale bar is a graphic that shows the map viewer how to translate between map units and real-world units. While historic maps often had verbal scales such as, “200 feet to the inch,” or “1 inch = 200 feet,” the modern convention is to use a graphic scale. When a paper map is scanned, shrunk, and put on a slide, or some other similar enlargement or reduction process takes place, a graphic scale will remain accurate. Scale bars are suitable for most large-scale maps but not suitable for maps where distance and area are not constant.