Cleisthenes served as chief archon in Athens in 525 BC. After the fall of the tyrant Hippias he established a democratic institution and he prepared Athens to face the Persian armies that threatened it.

He also introduced the practice of 'ostracism', or temporary banishment. His most important innovation was the basing of individual political responsibility on citizenship of a city rather than on membership of a clan.

In order to forestall strife between the traditional clans, which had led to the tyranny in the first place, he changed the political organisation from the four traditional tribes, which were based on family relations, into ten tribes according to their area of residence.

www link :
Biography