Rock music is a form of popular music with a prominent vocal melody, accompanied by guitar, drums, and bass. Many styles of rock music also use keyboard instruments such as organ, piano, mellotron, and synthesizers. Other instruments sometimes utilized in rock include harmonica, violin, flute, banjo, melodica, and timpani. Also, less common stringed instruments such as mandolin and sitar are used. Rock music usually has a strong back beat, and often revolves around the guitar, either solid electric, hollow electric, or acoustic.