The word "two-act" is used to describe any act played by two people. It has nothing to do with the number of scenes or acts of a drama. When two people present a "turn," it is called a two-act. It is a booking-office term—a word made necessary by the exigencies of vaudeville commerce.
If the manager of a theatre requires an acrobatic act to fill his bill and balance his show he often inquires for an acrobatic two-act. It may matter little to him whether the act plays in One or Full Stage—he wants an acrobatic act, and one presented by two people. If he requires any other kind of two-people-act, he specifies the kind of two-act of which he is in need.
On the other hand, if a performer asks an author to write a vaudeville two-act, an act of a certain definite character is usually meant and understood. For, among writers, the vaudeville two-act—or "act in One" as it is often called—has come to mean a talking act presented by two persons; furthermore, a talking act that has certain well-defined characteristics.
1. What a Vaudeville Two-Act Is
The most carefully constructed definition cannot describe even the simplest thing with satisfying exactness. But the human mind is so formed that it have a definition for a guide to learn anything is new. Therefore let us set up this dogmatic definition:
A pure vaudeville two-act is a humorous talking act performed by two persons. It possesses unity of the characters, is not combined with songs, tricks or any other entertainment form, is marked by compression, follows a definite form of construction, and usually requires from ten to fifteen minutes for delivery.
You have noticed that this definition is merely that of the monologue very slightly changed. It differs from it only in the number of persons required for its delivery. But, like many such verbal jugglings, the likeness of the two-act to the monologue is more apparent than real.
2. How the Two-Act Differs from the Monologue
Turn to the Appendix and read "The Art of Flirtation," by Aaron Hoffman. [1] It was chosen for publication in this volume as an example of the vaudeville two-act, for two reasons: First, it is one of the best vaudeville two-acts ever written; second, a careful study of it, in connection with "The German Senator," will repay the student by giving an insight into the difference in treatment that the same author gives to the monologue and the two-act.
[1] The Art of Flirtation," by Aaron Hoffman, has been used in vaudeville, on the burlesque stage, and in various musical comedies, for years and has stood the test of time.