Reforms introduced c.507 introduced a new tribal system, with ten tribes each divided into three trittyes, and all but the largest trittyes further divided into demes. This chapter examines the purpose and process of this restructuring, beginning from Kleisthenes and his mental map of Attika, and moving on to the tribes, trittyes, naukrariai, and organization of demes. Detailed analysis is included of Council quotas, lists of prytaneis, and ‘prytany trittyes’, as well as tribes in the army and fleet, tribe/trittys markers and horoi, tribal officials, meeting places, property, functions, tribe leaders and liturgists, and intertribal competitions.