In the years following these releases, original double albums among pop and rock artists increased, often released at the height of the artist's career. Notable examples include The Beatles' eponymous 1968 album, Elton John’s "Goodbye Yellow Brick Road" from 1973, and Led Zeppelin's "Physical Graffiti" from 1975, among others. Additionally, the rise of progressive rock at the time, which often involves complex and long tracks akin to classical music, and concept albums often made a second track necessary. The second best-selling double album and best-selling concept double album ever is Pink Floyd's "The Wall" with over 30 million copies (60 million units) worldwide.