Heavy metal music
Critics disagree over who can be thought of as the first heavy metal band. Most credit either Led Zeppelin or Black Sabbath, with American commentators tending to favour Led Zeppelin and British commentators tending to favour Black Sabbath, though many give equal credit to both. A few commentators—mainly American—argue for other groups including Iron Butterfly, Steppenwolf or Blue Cheer. Deep Purple, the third band in what is sometimes considered the "unholy trinity" of heavy metal (Black Sabbath, Led Zeppelin, and Deep Purple), despite being slightly older than Black Sabbath and Led Zeppelin, fluctuated between many rock styles until late 1969 when they took a heavy metal direction.
Heavy metal music
The combination of blues rock with psychedelic rock and acid rock formed much of the original basis for heavy metal. The variant or subgenre of psychedelic rock often known as "acid rock" was particularly influential on heavy metal; acid rock is often defined as a heavier, louder, or harder variant of psychedelic rock, or the more extreme side of the psychedelic rock genre, frequently containing a loud, improvised, and heavily distorted guitar-centered sound. Acid rock has been described as psychedelic rock at its "rawest and most intense," emphasizing the heavier qualities associated with both the positive and negative extremes of the psychedelic experience rather than only the idyllic side of psychedelia. American acid rock garage bands such as the 13th Floor Elevators epitomized the frenetic, heavier, darker and more psychotic sound of acid rock, a sound characterized by droning guitar riffs, amplified feedback, and guitar distortion, while the 13th Floor Elevators' sound in particular featured yelping vocals and "occasionally demented" lyrics. Frank Hoffman notes that: "Psychedelia was sometimes referred to as 'acid rock'. The latter label was applied to a pounding, hard rock variant that evolved out of the mid-1960s garage-punk movement. ... When rock began turning back to softer, roots-oriented sounds in late 1968, acid-rock bands mutated into heavy metal acts."