Aristotle's Poetics introduced the three unities, which are a theory of dramatic tragedy that emphasizes a play's unity of action, time, and place.
Unity of action
A play should have one main plotline
The action should be succinct and avoid subplots
Quality is more important than quantity
Unity of time
Unity of place
While many Greek tragedies observed these unities, some modern critics have rejected them. They argue that successful plays have violated the unities and that the unity of action should be the primary standard.