From WILL - Classics of the Phonograph -

Ballet Music

At the Paris Opera in the 19th Century, there was one rule: every grand opera had to have a ballet in the Third Act. No matter what the plot line, a reason must be found for someome in the story to do some dancing. If this rule caused problems for the likes of Wagner and Verdi, it resulted in some first rate ballet music. We’ll play some of this music, and you can dance, if you like.