(how certain works of literature fit into the ET formula) in this sentence what is the ET formula? What does this abbreviation mean?

The abbreviation ET refers to the literary form called Elizabethan Tragedy form. William Shakespeare used it exclusively in all of his works) though he was not the only writer making use of the form during the time. Prior to this time period most writers utilized or modeled their tragedies after the Aristotelian tragical outline. One of the primary differences between the two is that the Elizabethan form seems to issue forth a warning against tyrants, political corruption, and trechery at high levels. There may also be revenge elements that creep in (think Hamlet) where a hero tries to right a horrible injustice and it becomes their undoing.