Progressive rock originally referred to "classical rock" in which a band performed together with an orchestra, but the term's use broadened over time to include Miles Davis-style jazz fusion, some metal and folk rock styles, and experimental German bands. It does not refer to a single style but to an approach that combines elements of diverse styles. Jerry Ewing, editor of Prog Magazine, explains that "Prog is not just a sound, it's a mindset," and Dream Theater guitarist John Petrucci points out that it is defined by its very lack of stylistic boundaries. The advent of the concept album and the genre's roots in psychedelia led albums and performances to be viewed as combined presentations of music, lyrics, and visuals.