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Webster 1913 Edition
Tetralogy
Te-tral′o-gy
,Noun.
(Gr. Drama)
A group or series of four dramatic pieces, three tragedies and one satyric, or comic, piece (or sometimes four tragedies), represented consequently on the Attic stage at the Dionysiac festival.
☞ A group or series of three tragedies, exhibited together without a fourth piese, was called a trilogy.