Pentapolis

A pentapolis (from Greek πεντα- penta-, 'five' and πόλις polis, 'city') is a geographic and/or institutional grouping of five cities.

Cities in the ancient world probably formed such groups for political, commercial and military reasons, as happened later with the Cinque Ports in England.

The Pentapolis on the Adriatic was part of the Exarchate of Ravenna , an administrative unit of the Byzantine Empire .
Red: The Pentapolis. Orange: Other cities of the Exarchate.