The Three Unities were conventions which ancient Greek playwrights were expected to adhere to. Every play was to adhere to these rules, according to their originator, Aristotle. However, many playwrights, beginning with Aeschylus (?) began to break these conventions. The unities were:

1. The Unity of Time: The play was to take place during a single day.
2. The Unity of Space: The play must take place in a single location.
3. The Unity of Action: All action within the play was to be directed towards a single overarching idea.

These ideas were revived in 17th century France: see Classicism.

See also History of theater and Theatre technique