Bacon, Helen H. “The Chorus in Greek Life and Drama.” Arion: A Journal of Humanities and the Classics, vol. 3, no. 1, 1994, pp. 6–24. JSTOR, http://www.jstor.org/stable/20163562.

• The Athenians considered plays, regardless of genre, to be choruses. They also considered choruses to be public services.
• Poets had to ask magistrates "for a chorus" before they could produce their work. If "granted a chorus," they would be sponsored by a wealthy patron as part of their civic duties. The citizen would be called khorēgos ("chorus-leader")
• Participants in state-sponsored choruses could be exempt from military service
• Aristotle believed that tragedy was believed to have originated with the dithyramb, but we don't know much about them. Consisted of costumed groups of 50 each, 10 for men, 10 for boys, which represented Athens' 10 tribes.
• These dithyrambic groups performed traditional songs and dances, and sometimes their leaders would improvise solos, so as to be an individual that the chorus as a group could interact with.
• Dithyrambs only had men.
• Choruses rarely leave the stage.