According to cartometric scholars, the early normal portolan chart typically used three differing map scales - one for the Atlantic region, another for the Mediterranean Sea, and a third for the Black Sea, with the result that the distances in the Atlantic were understated and in the Black Sea relatively exaggerated.. This might be an error interpreting data, or deliberate (as the Black Sea was frequently sailed by merchants, and needed to be "zoomed" in for detail). Portolan charts usually have a small distance scale drawn somewhere on the map - usually a small bar with spaced lines and dots. However, this scale is usually unlabelled and its correspondence to actual distance measures (leagues, miles, etc.) left unspecified. The scale bar drawn in a normal portolan seems to correspond best with the Mediterranean scale.