Part One - the exposition, introduces the main characters, and begins the action.
Part Two - the development, continues the action and introduces complications.
Part Three - the crisis/ climax, brings everything to a head. In this part , a change of direction occurs or understanding is precipitated.
Part Four - includes further developments leaving inevitably to
Part Five - in which the final crisis of action or revelation and resolution are explained.
Source: Helen McCulloch, M.A., Dip Ed, & Gary K. Carey, M.A,. (2000). Shakespeare's Othello. USA; IDG Books Worldwide, Inc.
