He also made a law, to the effect that no man should be driven out of the city unless there were six thousand votes in favor of his exile. These votes were given in a strange way.

When a man was so generally disliked that his departure seemed best, all the Athenians assembled in the market place. Then each voter received a shell (Greek, ostrakon), and dropped it into a place made for that purpose. All in favor of banishment wrote upon their shells the name of the man they wished to exile. The others left theirs blank.

When all the votes had thus been cast, the shells were carefully counted, and, if six thousand bore the name of the same man, he was driven out of the city, or ostracized, as it was called from the name of the shell, for ten years.

From: H. A. Guerber, The Story of the Greeks; edited for this online publication, by ELLOPOS BLOG