• During the 1920s, genres such as the blues and so-called hillbilly music existed only on the margins of the popular music marketplace
• These styles grew out of southern folk music traditions, were shaped by the migration of millions of southerners from the country to the city, and eventually came to exert a profound influence on the development of American popular music.
• Record companies targeted new audiences between World War I and World War II (1918–40). New markets emerged for race and hillbilly music.
• The phonograph and radio helped increase the popularity of race and hillbilly music.
• The Great Depression marked the end of an important period in the development of American popular music and brought about many changes.