Why do some music styles gain mass popularity while others thrive in small niches? This book explores this question and reveals the attributes that together explain the growth of twentieth-century American popular music. Drawing on a vast array of examples from sixty musical styles—ranging from rap and bluegrass to death metal and South Texas polka, and including several created outside the United States—the book uncovers the shared grammar that allows us to understand the cultural language and evolution of popular music. The book discovers four dominant forms—avant-garde, scene-based, industry-based, and traditionalist—and two dominant trajectories that describe how American pop music genres develop. Outside the United States there exists a fifth form: the government-purposed genre, which the book examines in the music of China, Serbia, Nigeria, and Chile. Offering a rare analysis of how music communities operate, the book looks at the shared obstacles and opportunities creative people face and reveals the ways in which people collaborate around ideas, artworks, individuals, and organizations that support their work.