While original scores are used throughout Supernatural DVD episodes, another important aspect of the series' music is classic rock, which creator Eric Kripke threatened to quit over when the network would not allow its inclusion. Most of the songs are from Kripke's private collection Sons of Anarchy DVD, although one of his favorite bands—Led Zeppelin—is too expensive to use. Some episode titles are references to Led Zeppelin songs. The series has featured such bands as Blue Öyster Cult and AC/DC on more than one occasion. Multiple songs are usually Lost DVD used throughout each episode, and accompany "The Road So Far" sequences before select episodes that highlight a montage of past events. Although Kripke prefers to keep a fine line between the score and songs Bones DVDused, sometimes Lennertz and Gruska are required to write short sections of rock-like music to fill fifteen- to twenty-second gaps, as it would be too costly 24 Hours DVD to acquire song rights.