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.
Acid rock
By the early 1970s, bands such as Deep Purple, Led Zeppelin and Black Sabbath combined the loud, raw distortion of acid rock with occult lyrics, further forming a basis for the genre now known as "heavy metal". At a time when rock music began to turn back to roots-oriented soft rock, many acid rock groups instead evolved into heavy metal bands. As its own movement, heavy metal music continued to perpetuate characteristics of acid rock bands into at least the 1980s, and traces of psychedelic rock can be seen in the musical excesses of later metal bands. In the 1990s, the stoner metal genre combined acid rock with other hard rock genres such as grunge, updating the heavy riffs and long jams found in the acid rock and psychedelic-influenced metal of bands such as Black Sabbath, Blue Cheer, Hawkwind, and Blue Öyster Cult.