The relationship between music and theater can be traced back thousands of years, but the genre known as musical theater is a more recent invention. Ancient Greek playwrights often employed a group of singers or chanters known as a "Greek chorus." This chorus of actors would be used as a sounding board for the main characters, or act as the conscience of the audience. Occasionally professional musicians and singers would also perform onstage for the audience's entertainment or in the context of the drama, such as a wedding or funeral scene. Eventually some dramatists and composers collaborated on projects involving both drama and music. This finally led to the development of musical operas, the immediate forerunners of modern musical theater productions.
Musical theater productions can have comical or dramatic themes, with professional composers working closely with playwrights on the story's arc. Using Shakespeare's comedy Taming of the Shrew as its basic plot, for example, composer Cole Porter's musical play Kiss Me, Kate combines elaborate musical productions with a dramatic storyline about two actors reconciling their failed romance behind the scenes. Irving Berlin's musical Annie Get Your Gun uses the real-life biography of sharpshooter Annie Oakley as the plot for a romantic show business tale. Musical theater often uses music to explore the inner thoughts or motivations of the characters.
Before the arrival of radio, film or television, musical theater was one of the most popular forms of entertainment for the masses. Music halls in Great Britain and larger cities in the US would often feature individual musicians performing one after another, but there was rarely a sense of continuity between the acts. Early musical theater productions such as Showboat demonstrated that a dramatic production could be successfully accentuated with songs written specifically for the theme or plot line. If a character wanted to demonstrate his or her sadness, for example, musical theater allowed him or her to sing an emotional ballad, such as "Can't Help Loving That Man of Mine."
Modern musical theater productions often use state-of-the-art stage technology and first-rate composers to create spectacular productions. Musical theater also lends itself to film, since choreographers such as Busby Berkeley could stage elaborate musical production numbers not possible on a traditional stage. When dancer and singer Gene Kelly breaks into the song "Singin' in the Rain," for example, the audience is supposed to understand he is not literally dancing all over the rainy streets of New York City, but he is expressing his inner emotions after meeting the love of his life, played by Debbie Reynolds. In other musical theater productions, the cast will often break into a spontaneous dance number in order to support a main character. Many musical theater productions also allow performers to sing duets, solos and rousing crowd pleasers often referred to as "11 o'clock songs;" the usual time such songs are performed during Broadway productions.
While other comedic or dramatic plays will use music occasionally in context, musical theater generally gives equal weight to the "book," or dramatic plot, and the music. Some songs may become popular hits, while others may only serve to move the plot line or explore a character's innermost thoughts. Many popular musicals such as Phantom of the Opera, Cats and A Chorus Line can remain in continuous production for many years, while others may not last more than a few performances before closing. Others have become successful motion pictures after their initial runs on Broadway or London's West End.