structural:reformatting, stylistic:tone
Origins: late 1960s and early 1970s

Critics disagree over who can be thought of as the first heavy metal band. Most credit the British bands Led Zeppelin and 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. Deep Purple, the third band in what is sometimes considered the "unholy trinity" of heavy metal along with Led Zeppelin and Black Sabbath, fluctuated between many rock styles until late 1969 when they took a heavy metal direction. A few commentators – mainly American – argue for other groups, including Iron Butterfly, Steppenwolf, Blue Cheer, or Vanilla Fudge, as the first to play heavy metal.

In 1968, the sound that would become known as heavy metal began to coalesce. That January, San Francisco band Blue Cheer released a cover of Eddie Cochran 's classic " Summertime Blues " as a part of their debut album, Vincebus Eruptum, and many consider it to be the first true heavy metal recording. The same month, Steppenwolf released their self-titled debut album, on which the track " Born to Be Wild " refers to "heavy metal thunder" in describing a motorcycle. In July, the Jeff Beck Group, whose leader had preceded Page as The Yardbirds' guitarist, released its debut record, Truth, which featured some of the "most molten, barbed, downright funny noises of all time", breaking ground for generations of metal ax-slingers. In September, Page's new band, Led Zeppelin, made its live debut in Denmark (but were billed as The New Yardbirds). The Beatles ' self-titled double album, released in November, included " Helter Skelter ", then one of the heaviest-sounding songs ever released by a major band. The Pretty Things ' rock opera S.F. Sorrow, released in December, featured "proto heavy metal" songs such as "Old Man Going" and "I See You". Iron Butterfly 's 1968 song " In-A-Gadda-Da-Vida " is sometimes described as an example of the transition between acid rock and heavy metal or the turning point in which acid rock became "heavy metal", and both Iron Butterfly's 1968 album In-A-Gadda-Da-Vida and Blue Cheer's 1968 album Vincebus Eruptum have been described as laying the foundation of heavy metal and greatly influential in the transformation of acid rock into heavy metal.

In this counterculture period, MC5, who began as part of the Detroit garage rock scene, developed a raw, distorted style that has been seen as a major influence on the future sound of both heavy metal and later punk music. The Stooges also began to establish and influence a heavy metal and later punk sound, with songs such as " I Wanna Be Your Dog ", featuring pounding and distorted heavy guitar power chord riffs. Pink Floyd released two of their heaviest and loudest songs to date, " Ibiza Bar " and " The Nile Song ", the latter of which being regarded as "one of the heaviest songs the band recorded." King Crimson 's debut album started with " 21st Century Schizoid Man ", which was considered heavy metal by several critics.

In January 1969, Led Zeppelin's self-titled debut album was released and reached No. 10 on the Billboard album chart. In July, Led Zeppelin and a power trio with a Cream-inspired, but cruder sound, called Grand Funk Railroad played the Atlanta Pop Festival. That same month, another Cream-rooted trio led by Leslie West released Mountain, an album filled with heavy blues rock guitar and roaring vocals. In August, the group – now itself dubbed Mountain – played an hour-long set at the Woodstock Festival, exposing the crowd of 300,000 people to the emerging sound of heavy metal. Mountain's proto-metal or early heavy metal hit song " Mississippi Queen " from the album Climbing! is especially credited with paving the way for heavy metal and was one of the first heavy guitar songs to receive regular play on radio. In September 1969, the Beatles released the album Abbey Road containing the track " I Want You (She's So Heavy) ", which has been credited as an early example of or influence on heavy metal or doom metal. In October 1969, British band High Tide debuted with the heavy, proto-metal album Sea Shanties.

Led Zeppelin defined central aspects of the emerging genre, with Page's highly distorted guitar style and singer Robert Plant 's dramatic, wailing vocals. Other bands, with a more consistently heavy, "purely" metal sound, would prove equally important in codifying the genre. The 1970 releases by Black Sabbath (Black Sabbath, which is generally accepted as the first heavy metal album, and Paranoid) and Deep Purple (Deep Purple in Rock) were crucial in this regard.

Birmingham 's Black Sabbath had developed a particularly heavy sound in part due to a work accident in which guitarist Tony Iommi lost the ends of two fingers. Unable to play normally, Iommi had to tune his guitar down for easier fretting and rely on power chords with their relatively simple fingering. The bleak, industrial, working-class environment of Birmingham, a manufacturing city full of noisy factories and metalworking, has itself been credited with influencing Black Sabbath's heavy, chugging, metallic sound – and the sound of heavy metal in general.

Deep Purple had fluctuated between styles in its early years, but by 1969, vocalist Ian Gillan and guitarist Ritchie Blackmore had led the band toward the developing heavy metal style. In 1970, Black Sabbath and Deep Purple scored major U.K. chart hits with " Paranoid " and " Black Night ", respectively. That same year, two other British bands released debut albums in a heavy metal mode: Uriah Heep with... Very 'Eavy... Very 'Umble and UFO with UFO 1. Bloodrock released their self-titled debut album, a collection of heavy guitar riffs, gruff style vocals and sadistic and macabre lyrics. The influential Budgie brought the new metal sound into a power trio context, creating some of the heaviest music of the time. The occult lyrics and imagery employed by Black Sabbath and Uriah Heep would prove particularly influential; Led Zeppelin also began foregrounding such elements with its fourth album, released in 1971. In 1973, Deep Purple released the song " Smoke on the Water ", whose iconic riff is usually considered as the most recognizable one in "heavy rock" history, as a single of the classic live album Made in Japan.

On the other side of the Atlantic, the trendsetting group was Grand Funk Railroad, who was described as "the most commercially successful American heavy-metal band from 1970 until they disbanded in 1976, established the Seventies success formula: continuous touring." Other influential bands identified with metal emerged in the U.S. such as Sir Lord Baltimore (Kingdom Come, 1970), Blue Öyster Cult (Blue Öyster Cult, 1972), Aerosmith (Aerosmith, 1973) and Kiss (Kiss, 1974). Sir Lord Baltimore's 1970 debut album and both Humble Pie 's debut and self-titled third album were among the first albums to be described in print as "heavy metal", with As Safe As Yesterday Is referred to by the term "heavy metal" in a 1970 review in Rolling Stone magazine. Various smaller bands from the U.S., U.K. and Continental Europe – including Bang, Josefus, Leaf Hound, Primeval, Hard Stuff, Truth and Janey, Dust, JPT Scare Band, Frijid Pink, Cactus, Irish Coffee, May Blitz, Captain Beyond, Toad, Granicus, Iron Claw, and Yesterday's Children – though lesser known outside of their respective scenes, proved to be greatly influential on the emerging metal movement. In Germany, Scorpions debuted with Lonesome Crow in 1972. Blackmore, who had emerged as a virtuoso soloist with Deep Purple's highly influential album Machine Head (1972), left the band in 1975 to form Rainbow with Ronnie James Dio, singer and bassist for blues rock band Elf and future vocalist for Black Sabbath and heavy metal band Dio. Rainbow with Ronnie James Dio would expand on the mystical and fantasy -based lyrics and themes sometimes found in heavy metal, pioneering both power metal and neoclassical metal. These bands also built audiences via constant touring and increasingly elaborate stage shows.

There are arguments about whether these and other early bands truly qualify as "heavy metal" or simply as "hard rock". Those closer to the music's blues roots or placing greater emphasis on melody are now commonly ascribed the latter label. AC/DC, which debuted with High Voltage in 1975, is a prime example. The 1983 Rolling Stone encyclopedia entry begins, "Australian heavy-metal band AC/DC..." Rock historian Clinton Walker wrote, "Calling AC/DC a heavy metal band in the seventies was as inaccurate as it is today.... were a rock 'n' roll band that just happened to be heavy enough for metal." The issue is not only one of shifting definitions, but also a persistent distinction between musical style and audience identification; Ian Christe describes how the band "became the stepping-stone that led huge numbers of hard rock fans into heavy metal perdition".

In certain cases, there is little debate. After Black Sabbath, the next major example is Britain's Judas Priest, which debuted with Rocka Rolla in 1974. In Christe's description,

Black Sabbath's audience was... left to scavenge for sounds with similar impact. By the mid-1970s, heavy metal aesthetic could be spotted, like a mythical beast, in the moody bass and complex dual guitars of Thin Lizzy, in the stagecraft of Alice Cooper, in the sizzling guitar and showy vocals of Queen, and in the thundering medieval questions of Rainbow.... Judas Priest arrived to unify and amplify these diverse highlights from hard rock's sonic palette. For the first time, heavy metal became a true genre unto itself.

Though Judas Priest did not have a top 40 album in the United States until 1980, for many it was the definitive post-Sabbath heavy metal band; its twin-guitar attack, featuring rapid tempos and a non-bluesy, more cleanly metallic sound, was a major influence on later acts. While heavy metal was growing in popularity, most critics were not enamored of the music. Objections were raised to metal's adoption of visual spectacle and other trappings of commercial artifice, but the main offense was its perceived musical and lyrical vacuity: reviewing a Black Sabbath album in the early 1970s, Robert Christgau described it as "dull and decadent... dim-witted, amoral exploitation."