Art rock (or progressive rock) is the term for a style that is musically more ambitious and complex than most other types of rock. It can involve the use of unusual time signatures and rhythms as well as the incorporation of classical elements into a rock context. Most art rock groups came from the UK and include bands such as the Moody Blues, Yes, Emerson Lake  and Palmer, Procol Harum, Pink Floyd, King Crimson, and Renaissance. Sometimes these groups would play rock style versions of classical pieces (Emerson Lake and Palmer's Pictures at an Exhibition, originally by Russian Modest Musorgsky is a good example). Other times they teamed up with classical musicians to create rock pieces with an orchestral flavor (The Moody Blues Knights in White Satin). Usually improvisation took a back seat to elaborate arrangements in art rock songs and the synthesizer often played an important role. In the United States, musicians like Frank Zappa created music that was virtually uncategorizable. His album Hot Rats (1969) includes several instrumentals that are very complex and unusual.

Pink Floyd's Dark Side of the Moon (album cover pictured above) is a great example of  progressive rock.  The album's songs are connected seamlessly and include music that is much more complex than typical rock tracks. Consider the song "Money." It begins with a rhythm created by cash registers (an example of musique concrete, an approach to classical composition popular in the early 20th century). The rhythm is 7/4---very unusual for rock music.