Heavy metal (or simply metal) is a genre of rock music that developed in the late 1960s and early 1970s, largely in the United Kingdom and to a lesser extent in the United States. With roots in blues rock, psychedelic rock, and acid rock, heavy metal bands developed a thick, massive sound, characterized by distortion, extended guitar solos, emphatic beats, and loudness. The lyrics and performances are sometimes associated with aggression and machismo.
During the 1980s, glam metal became popular with groups such as Bon Jovi and Mötley Crüe. Underground scenes produced an array of more aggressive styles: thrash metal broke into the mainstream with bands such as Metallica, Slayer, Megadeth, and Anthrax, while other extreme subgenres such as death metal and black metal remain subcultural phenomena. Since the mid-1990s, popular styles have expanded the definition of the genre. These include groove metal and nu metal, the latter of which often incorporates elements of grunge and hip hop.
Heavy metal is traditionally characterized by loud distorted guitars, emphatic rhythms, dense bass-and-drum sound, and vigorous vocals. Heavy metal subgenres variously emphasize, alter, or omit one or more of these attributes. The New York Times critic Jon Pareles writes, "In the taxonomy of popular music, heavy metal is a major subspecies of hard-rock—the breed with less syncopation, less blues, more showmanship and more brute force." The typical band lineup includes a drummer, a bassist, a rhythm guitarist, a lead guitarist, and a singer, who may or may not be an instrumentalist. Keyboard instruments are sometimes used to enhance the fullness of the sound. Deep Purple's Jon Lord played an overdriven Hammond organ. In 1970, John Paul Jones used a Moog synthesizer on Led Zeppelin III; by the 1990s, in "almost every subgenre of heavy metal" synthesizers were used.
The electric guitar and the sonic power that it projects through amplification has historically been the key element in heavy metal. The heavy metal guitar sound comes from a combined use of high volumes and heavy distortion. For classic heavy metal guitar tone, guitarists maintain gain at moderate levels, without excessive preamp or pedal distortion, to retain open spaces and air in the music; the guitar amplifier is turned up loud to produce the characteristic "punch and grind". Thrash metal guitar tone has scooped mid-frequencies and tightly compressed sound with multiple bass frequencies. Guitar solos are "an essential element of the heavy metal code ... that underscores the significance of the guitar" to the genre. Most heavy metal songs "feature at least one guitar solo", which is "a primary means through which the heavy metal performer expresses virtuosity". Some exceptions are nu metal and grindcore bands, which tend to omit guitar solos. With rhythm guitar parts, the "heavy crunch sound in heavy metal ... [is created by] palm muting" the strings with the picking hand and using distortion. Palm muting creates a tighter, more precise sound and it emphasizes the low end.
The lead role of the guitar in heavy metal often collides with the traditional "frontman" or bandleader role of the vocalist, creating a musical tension as the two "contend for dominance" in a spirit of "affectionate rivalry". Heavy metal "demands the subordination of the voice" to the overall sound of the band. Reflecting metal's roots in the 1960s counterculture, an "explicit display of emotion" is required from the vocals as a sign of authenticity. Critic Simon Frith claims that the metal singer's "tone of voice" is more important than the lyrics.