http://agathe.gr/democracy/practice_of_ostracism.html
Ostracism Soon after their victory over the Persians at the battle of Marathon in 490 B.C., the Athenians began the practice of ostracism, a form of election designed to curb the power of any rising tyrant ... Ostracism Soon after their victory over the Persians at the battle of Marathon in 490 B.C., the Athenians began the practice of ostracism, a form of election designed to curb the power of any rising tyrant. ... The procedure of ostracism was simple. ... More than 4,000 ostraka bearing Megakles' name were found in one deposit in the Kerameikos (the potters' quarter of Athens) and have been associated with the ostracism of 486 B.C., although the rude comments that accompany his name on some of these ostraka concentrate on his morals rather than on his tyrannical tendencies.