The early 20th century also saw the growth of Broadway, a group of theaters specializing in musicals. Broadway became one of the preeminent locations for musical theater in the world, and produced a body of songs that led Donald Clarke to call the era, the "golden age of songwriting". The need to adapt enjoyable songs to the constraints of a theater and a plot enabled and encouraged a growth in songwriting and the rise of composers like George Gershwin, Vincent Youmans, Irving Berlin and Jerome Kern. These songwriters wrote songs that have remained popular and are today known as the Great American Songbook.