Aeschylus and Tragic Drama by George Thomson

The tragic performances were the principal event at an annual festival at which a prize was awarded for the best drama of the year. 

Each competitor was required to submit four plays a “tetralogy” as it was called, consisting of three tragedies ( a Trilogy) followed by a Satyr play. The Satyr play was burlesque, so called because it had a chorus of satyrs. 

These mere mythical creatures half man half beast, representing the unsophisticated savage men as seen by the Athenians. (They may be compared to the bird man Papageno in Mozarts Magic Flute).

Let us take as an example the great tetralogy of Aeschylus, the Oresteia. The story taken from mythology, contains many primitive features such as the ancestral curse but at the end these are all relegated to the past

In the first play, Agamemnon is murdered on his return from Troy by his wife Clytemnestra. In the second she is murdered by her son Orestes at the command of Apollo. In the third after being purified by Apollo and persecuted by his mothers avenging spirits ,the furies, Orestes is brought to trial and acquitted before a court of justice founded for this purpose by Athena, goddess of democratic Athens.

She resolves the conflict between Apollo and the Furies by inviting them to supervise the new court.The Reign of Law had began.

At the end of the trilogy the story is revealed to us in retrospect as a symbol of man's struggle to raise himself out of savagery into civilization.The satyr play dealt with the wanderings on Menelaus after the fall of Troy as a romantic lighthearted  parallel to his brothers tragic homecoming.

Aeschylus was a democrat and a Pythagorean. He believed that conflict between tribal custom, represented the drama of the furies and aristocratic privilege represented by Apollo, had been resolved in democracy which accordingly he regarded as the fusion of opposites in the mean. 

In the trilogy therefore which represented the offense, the counter offense and the reconciliation, he created a dramatic form provided a perfect vehicle for dialectical movement of thought.

Source : the Human Essence