Thrash metal (sometimes referred to simply as thrash) 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 metal lyrics also often deal with social issues using visceral and blunt language, an approach which partially overlaps with the hardcore genre. A lot of times the vocals are clean cut. The "Big Four" of American thrash metal are Slayer, Megadeth, Metallica, and Anthrax, who simultaneously created and popularized 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, and the attitude of the punk movement, thus 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, high-register guitar solos, and aggressive vocals. 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 Motörhead's Lemmy.
Lyrical themes typical to thrash metal include anti-authoritarian thoughts, a focus on the balance between life and death, and critiques of religion and modern society. A focus on warfare and apocalyptic themes is also not uncommon. That said, in contrast to many extreme metal genres to follow, thrash metal sometimes focused on positive social issues, for instance environmentalism as was in the case of the band Nuclear Assault.