Syd Field was a screenwriting guru, most famous for his articulation of the typical three-act structure. He is the author of [Screenplay (1979)](http://www.amazon.com/dp/0385339038/?tag=johnaugustcom-20), among other books on screenwriting.
In Field’s structure, the first act, which takes approximately one quarter of the film’s runtime, sets up the conflict. A plot point thrusts the main character into the second act, which depicts the character’s struggle to achieve his or her goal. This act takes up half of the movie’s runtime. The final quarter of the film is the third act, which features the climactic struggle in which the character either achieves or fails to achieve the goal; the third act then continues through the aftermath of this climax.
Field passed away in 2013.