Thrash metal is a subgenre of heavy metal music, one of the extreme metal subgenres that is characterised by high speed riffing and aggression. Thrash metal songs typically use fast, percussive and low-register guitar riffs, overlaid with shredding-style lead work. Thrash song lyrics sometimes express views which prefers to attempt an objective critique of society. Thrash metal lyrics also often deal with social issues using visceral and blunt language, an approach which partially overlaps with the hardcore genre. The so-called "Big Four (of thrash)" are Megadeth, Slayer, Metallica, and Anthrax, who essentially simultaneously created the genre in the early 1980s.
The origins of thrash metal are generally traced to the late 1970s and early 1980s, when a number of bands began incorporating the sound of the New Wave of British Heavy Metal, creating a new genre and developing into a separate movement from punk rock and hardcore. This genre is much more aggressive compared to its relative, speed metal. There is often significant crossover from one metal category to another, and some bands use musical influences from non-metal genres.
Thrash metal features a number of fast and rapid tempos, low-register, quick, complex guitar riffs, and high-register guitar solos. Thrash rhythm guitar playing is characterized by palm muting and staccato used with a heavily distorted tone and tightly controlled riffs to create a "chugging" sound.
Thrash guitar solos are almost exclusively played at high speed, as they are usually characterized by shredding, and use techniques such as sweep picking, legato phrasing, alternate picking, string skipping, and two-hand tapping. Thrash lead guitarists are rooted in the New Wave of British Heavy Metal movement.
The speed and pacing of the songs is what defines thrash metal. The music tends to have a visceral, propellant feel to it due to its drumming style, most commonly utilizing the snare drum on the 1/2 beat, or the 2nd and 4th beats of the measure . Frantic bass drum use is also common. Thrash drummers often use two bass drums. Some thrash drummers are revered as some of the "best drummers in rock music". 
Due to the genre's high speed, many thrash bassists use a pick to keep up with the other instruments. However, some prominent bassists within thrash used their fingers, including Frank Bello, Greg Christian and the late Cliff Burton.  Several bassists use an overdriven or distorted bass tone, an approach popularized by Burton and Motorhead's Lemmy.
Lyrical themes worked in thrash include gore, life and death, religion and society. For it working with warfare, brutal feelings supposedly hidden into human unconsciousness and visions of a possible future [[collapse of civilization] In contrast to many extreme metal genres to follow, thrash metal often focused on positive social issues, for instance environmentalism as was in the case of the band Nuclear Assault.
1981 is seen as a critical year for the development of thrash metal, but work prior to 1981 helped shape the genre. Queen's 1974 song "Stone Cold Crazy" is cited as an early precursor of the thrash metal sound.; Metallica would win a Grammy Award in 1991 for their recording of the song.
Soon after "The Beast Within" was released, a Southern California band named Leather Charm would write the song "Hit the Lights". Leather Charm was a short-lived band that would soon break up. The primary songwriter for Leather Charm, James Hetfield, joined Metallica when he found an article in the paper that Lars Ulrich posted looking for a vocalist, guitarist, and bassist. The song, "Hit the Lights" was released by Metallica.
The European thrash scene that began in early 82 was almost exclusively influenced by the most aggressive music both Germany and England were producing at that time. NWOBHM Bands such as Tank, Raven and Venom, along with homegrown metal exports Accept, motivated musicians from eastern Europe to start bands of their own and produced German teutonic thrash exports such as Sodom, Kreator and Destruction.
Metallica released the No Life 'Til Leather demo in July 1982, and was the second band to release a studio LP, after Venom. It was titled Kill 'Em All and released in July 1983. Shortly following the release of Kill 'Em All, Slayer released their debut full length album entitled Show No Mercy, in December 1983.
The popularity of thrash metal increased in 1984 with the release of Metallica's Ride the Lightning, Anthrax's Fistful of Metal, Overkill's self-titled EP and Slayer's Haunting the Chapel. This led to a heavier sounding form of thrash, which was reflected in Exodus's Bonded by Blood and Slayer's Hell Awaits. In 1985, the German band Kreator released their debut album Endless Pain and the Brazilian band Sepultura released their EP Bestial Devastation. Megadeth, which was formed by former Metallica guitarist Dave Mustaine, released their debut album Killing Is My Business... And Business Is Good! in 1985.
Some of the most influential thrash albums were released in 1986. Dark Angel Released its second album Darkness Descends in 1986, featuring extremely 'over the top' style, with lengthy, extremely fast (some over 300 bpm), violent and gory songs. Slayer, regarded as one of the most sinister thrash metal bands from the early 1980s released Reign in Blood, considered by many a highly influential album (according to All Music Guide, this record has inspired the entire death metal genre). Kreator released Pleasure to Kill, which would later be an influence on the death metal genre. 
Metallica's 1986 album Master of Puppets, was one of the first thrash metal albums to receive critical acclaim and commercial success. In November of the same year, Megadeth released Peace Sells... But Who's Buying?, which proved to be the band's commercial and critical breakthrough. Considered to be a landmark thrash metal album, All Music Guide cited Peace Sells... But Who's Buying? as "One of the most influential metal albums of its decade, and certainly one of the few truly definitive thrash albums".
Some bands combined speed and thrash metal. Megadeth, Helstar, Testament, and Heathen were known for flashy lead guitar work. Watchtower's 1985 release Energetic Disassembly showcased much more progressive work, using very odd time signatures and jazz influenced song structures.
The late 1980s showed thrash metal experiencing a radical change. Little by little, thrash metal became more musical as opposed to lyrically oriented. Though the genre was already known for being technical, it would be pushed to a whole new extreme that would result in many more bands forming.
In 1987, Anthrax released their album Among the Living, which bore similarities to their two previous releases: Fistful of Metal and Spreading the Disease, with fast and heavy guitars and pounding drums. Anthrax's songs can be considered slightly more "melodic" when compared to other thrash metal bands of the era, due to their upbeat and catchy riffs.
Testament would release their debut album, The Legacy, that same year. The musical tone of Testament generally emphasized the more progressive elements of thrash metal. The lyrics on this album especially were about the occult and Satanic topics that would influence the lyrics of death metal. Death Angel took a similar pro-thrash approach with their 1987 debut, The Ultra-Violence, which featured more operatic lyrics influenced by classic rock acts like Queen and NWOBHM bands such as Iron Maiden.
In 1988, Suicidal Tendencies, who had previously been a straightforward punk band, released their major label debut How Will I Laugh Tomorrow When I Can't Even Smile Today. This album had very thrashy guitar riffs and an overall very metal oriented sound, with much more complicated song structures than on their previous albums, but the band still stayed true to their roots as a band in that the songs were very melodic and had catchy choruses .
By 1988 or so, the genre was quite saturated with new bands, but classic albums would still be recorded and released. Sepultura's third album, Beneath the Remains (1989) earned them some mainstream appeal as it appeared on Roadrunner records. Testament continued through the late 1980s with The New Order (1988) and Practice What You Preach (1989), both albums showing the band was continuing to grow musically and almost gaining Testament the same level of popularity as the "Big Four" of thrash: Metallica, Megadeth, Anthrax and Slayer. Vio-lence, a relative latecomer to the Bay Area thrash metal scene put out an acclaimed debut in Eternal Nightmare (1988), combining relentless riffage with a punk vocal delivery, resulting in one of the fastest, heaviest thrash albums of all time. Canadian thrashers Annihilator would release their highly technical debut album Alice in Hell (1989) which received much praise due to its fast riffs and virtuostic guitar solos. Sadus was another band influenced by the Thrash Metal genre, featuring a very strong sound which was primarily caused by the fretless bass of Steve DiGiorgio. Meanwhile in Germany, Sodom released "Agent Orange" and Kreator would release "Extreme Aggression". Both albums hit the scene in 1989 and are highly regarded as thrash metal classics by fans all around the world.
Older bands continued to record classic albums though. Megadeth released So Far, So Good... So What! in 1988, while Metallica's album ...And Justice for All of the same year spawned the band's first video, the World War I-themed song "One". The extremely complex song structure of "One" has made it a pioneering song among progressive metal bands, including Dream Theater, which has cited Metallica as a major influence.
Among the thrash albums released in 1990 included Megadeth's Rust in Peace, Anthrax's Persistence of Time, Slayer's Seasons in the Abyss, Suicidal Tendencies' Lights...Camera...Revolution!, Testament's Souls of Black, Annihilator's Never, Neverland and Kreator's Coma of Souls. All of those albums were commercial high points for the aforementioned artists. Most of these bands embarked on the Clash of the Titans tour that year.
In the 1990s, as some bands explored more innovative directions, bands that played classic thrash metal were seen as throw-backs. The 1992 album by Iced Earth, Night of the Stormrider combined power-metal and thrash metal. Dark Angel's swansong album Time Does Not Heal was a technically challenging, progressive thrash with a number of complex riffs. Many bands, however, opted for a slower, more groove-oriented sound, including Machine Head (formed from Vio-Lence) and Pantera.
While alternative rock was the predominant genre of the 1990s, thrash metal managed to gain influence. The alternative rock band Primus (who featured ex-Possessed guitarist Larry LaLonde) blended Les Claypool's funky bass lines with thrash-influenced guitar riffs and fast tempo songs.
Many veteran thrash metal bands began changing to more accessible, radio-friendly styles. In 1991, Metallica released their self-titled album, which saw record album sales for the band. Later in the decade, the band released Load (1996), and ReLoad (1997), which both displayed minor blues and southern rock influences.
Megadeth and Anthrax also changed their sound during the 1990s for varying reasons. Megadeth took a more accessible route with their 1992 album Countdown To Extinction. Anthrax split with frontman Joey Belladonna and recruited Armored Saint singer John Bush whose vocals style was different from that of their previous vocalist. The first release from the new line-up was the 1993 album Sound of White Noise.
Testament released the mainstream and melodic The Ritual in 1992 before switching to a more death metal oriented sound that would follow throughout the 1990s. Annihilator switched to a more commercial style on Set the World on Fire and King of the Kill before returning to a more thrash-oriented style. Kreator began experimenting with industrial metal and goth starting with Renewal. Some bands, such as Slayer, did not adopt a more mainstream approach. Slayer's album Divine Intervention has a classic thrash sound.
Members of Sepultura, Annihilator, and many others performed on RoadRunner's United album in 2005-06. The first release of its kind combined vintage thrash talents with newer metal musicians who incorporate thrash styles. There has recently been older thrash metal bands that have reunited and put out new albums including Nuclear Assault who has just recently released their new album Third World Genocide. Recently the original line-up of the band Testament reunited and toured, there was also a live album and video released with the classic line-up playing called Live In London. They released a new album called "The Formation of Damnation" on April 29 2008.
Recently thrash (along with many genres of extreme metal previously considered dead by the mainstream media, but well alive to the underground metal scene) has seen a certain degree of resurgence of popularity, including (but not limited to) the younger audience of Generation Y. This is perhaps due in part to an increase in exposure to many forms of metal and classic rock, thanks to the publicity spotlighted on it by such mediums as internet radio, satellite radio, cable networks like VH1, MTV, and cover songs by newer bands of older metal classics.
Older thrash bands have continued to put out material, such as Sodom's self-titled album (2006), Megadeth's United Abominations (2007), Slayer's Christ Illusion (2006), Kreator's Enemy of God (2005), Exodus' The Atrocity Exhibition...Exhibit A (2007), Overkill's Immortalis (2007), Onslaught's Killing Peace (2007 and Testament's The Formation of Damnation (2008).
Many note that thrash metal is making a comeback due to the popularity of metalcore bands, many of which claim to be thrash metal bands though having very little in common with the genre. This is very similar to the 1980s, when the genre was created to combat the rising movement of glam metal bands, many of which were thrown of major labels to be replaced by thrash metal bands, then their popularity faded and were ignored by labels all together. The same cycle (as some claim) is starting to repeat itself, with newer bands like Municipal Waste, Evile and Warbringer getting signed to larger labels such as Century Media Records and Earache Records. Bands such as Merciless Death, Avenger of Blood, Fueled By Fire, Dekapitator, Meltdown, Bonded By Blood, and Insinnerator are also releasing demos and albums that represent the style of old-school thrash metal, either releasing them themselves (Insinnerator, Devastator), or are being signed to more independent labels such as Heavy Artillery (Merciless Death, Avenger of Blood). Fueled By Fire, along with fellow thrashers Hatchet, have been signed to Metal Blade Records.
Like many musical genres, thrash had its own regionally-based scenes, each of which had a slightly different sound. The four most well known of these scenes were:
East Coast (New York/New Jersey)Edit
The East Coast bands tended to be more punk and hardcore influenced than West Coast bands, with more emphasis on aggression and speed than technicality (though in the case of bands like Toxik this was not the case). Anthrax, Nuclear Assault, Overkill, Whiplash, Fantom Warior, as well as crossover acts S.O.D., M.O.D., Cro-Mags, and Agnostic Front were a few of the most prominent bands to come from the East Coast thrash scene.
Not one of the major thrash scenes, but a few famous bands have come out of Brazil like Sepultura and Sarcófago. Bands from Brazil are influenced by the likes of Slayer and have a lot of death metal riffs.
Thrash metal with even more punk elements than standard thrash is called crossover thrash or crossover for short. Its overall sound is more punk-influenced than traditional thrash metal, while more metal sounding than traditional hardcore punk and thrashcore. Crossover is often more aggressive and faster than traditional punk due to the heavier thrash metal influence.
Crossover incorporated slower paced thrash riffs mixed with breakdown riffs commonly used in hardcore and helped forge two derivatives, one known as groove metal (sometimes called post-thrash), and one known as metalcore. The former, groove metal, has more of a darker heavy metal aesthetic, including guitar solos, classical music influence, and blues roots from traditional metal, and staying close to its thrash metal roots, and the latter, metalcore, having more of a hardcore punk aesthetic, usually excluding guitar solos in favor of rhythmic guitars.
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