Butler is noted as being one of the first bassists to use a wah pedal and to down-tune his instrument (from the standard E-A-D-G to the lower C#-F#-B-E), as on Black Sabbath's "Master of Reality" album. This was to match Iommi who had started tuning his guitar to C# (a minor third down). During the band's Ozzy Osbourne era, Butler wrote almost all of the band's lyrics, drawing heavily upon his fascination with religion, science-fiction, fantasy and horror, and musings on the darker side of human nature that posed a constant threat of global annihilation.