Broadway musicals vary greatly in content, lyrics, score, costume, lighting and set design, but there are some common structural elements.
Overture
The swell of music heard after theater lights dim, but before the start of onstage action, is called the overture. The piece usually contains “many of the musical motifs and themes of the score, according to the Glossary of Technical Theatre Terms.
Opening Number
According to author Mark Steyn, the first song of a successful Broadway musical “defines the (main character’s) motivation, kick-starts the plot and communicates the spirit of the score and the staging.” The opening number sets the tone for the whole show and starts the “storytelling process” by grabbing the audience’s attention.
“I Want” Song
Musical history site Musicals 101 explains that an “I Want” song reveals character motivations and desires. The song typically appears early in the musical’s first act with one or more characters singing about “the key motivating desire that will propel everyone through the remainder of the show.”
“Eleven O’Clock Number”
Musicals 101 explains that an “Eleven O’clock Number” takes place during the middle of the second act. It may be a ballad, a lighthearted number or comedy song that “energizes the audience for the final scenes.”
Finale
A powerful closing number wraps up the show’s plot and leaves a strong impression on the audience. Frequently, the finale includes a reprise of the one of the show’s “emotion-packed” numbers.