From the early days of radio through the rise of television after World War II to the present, music has been used more and more to sell goods and establish brand identities. And since the 1920s, songs originally written for commercials have become popular songs, and songs written for a popular audience have become irrevocably associated with specific brands and products. Today, musicians move flexibly between the music and advertising worlds, while the line between commercial messages and popular music has become increasingly blurred.