The ChorusThe tragic chorus had at first twelve, and then fifteen members and was arranged in rectangular form like battle lines of hoplites (heavily-armed infantrymen):

The best dancers were in the row that had the audience on their left (here the bottom row). The chorus leader (koryphaios) was in the middle of this row. The next best dancers were in rear row, while the worst were hidden in the center.
The first 135 years of tragedy show a continual decline in the role of the chorus, both in the number of lines they sing and in their importance to the drama. The tragedies of Aeschylus are 40 to 50% choral - which is what you would expect from an early tragedian. Tragedy started out as purely a choral performance, until an actor was separated out from the chorus to engage in dialogue with the chorus, thereby making drama possible. Before 425 BC (with the exception of the Prometheus Bound) no tragedy is less than 20% choral. Some of the late plays of Euripides, however, are as low as 10% choral. Another indication of the less than integral role of the chorus late in the century is Aristotle’s comment that Agathon, whose plays have not survived, began writing generic choral songs that could be fitted into any tragedy. The choral songs were fast becoming no more than interludes, without any particular relevance to the action of the play. Perhaps the single most important factor in the decline of the chorus was the conflict between the increasing desire for realism in drama and traditional convention that required the chorus not to interfere in the action of the play. It became increasingly hard for the audience to accept the presence of the chorus, which witnessed gruesome murders, but could not intervene.
Although the actors were strictly professional, the members of chorus were amateurs recruited from the citizen body. The only exception was the, the chorus leader (koryphaios), who was a professional, and spoke lines of dialogue, when the chorus conversed with actors. Since participation in a chorus with its singing and dancing was very physically taxing, choruses were made up of young men. In the Great Dionysia only citizens could serve in the chorus. A choregos (‘producer’) who allowed foreigners or disenfranchised Athenians to be chorus members were fined 1000 drachmas and a citizen could forcibly remove a non-citizen chorus member right in the middle of the performance. The amateurishness of the chorus, contrasted with professionalism of the rest of the production, probably also contributed to the decline of the chorus’s role in drama.
Choral songs were usually organized into stanzas called strophe (‘turn’), antistrophe (‘turning the other way’), and epode (‘added song’). As is clear from their meaning, strophe and antistrophe are dance terms. These two stanzas corresponded to each other metrically. An ancient commentator on Euripides’ Hecuba (647) says: