Thrash metal (or simply thrash) is an extreme subgenre of heavy metal music characterized by its overall aggression and often fast tempo.The songs usually use fast percussive beats and low-register guitar riffs, overlaid with shredding-style lead guitar work. The lyrical subject matter often deals with criticisms of The Establishment and concern over the destruction of the environment, and at times shares a disdain for Christian dogma resembling that of their black metal counterparts. The language is typically direct and denunciatory, an approach borrowed from hardcore punk. The genre emerged in the early 1980s as musicians began fusing the double bass drumming and complex guitar stylings of the new wave of British heavy metal (NWOBHM) with the speed and aggression of hardcore punk. Philosophically, thrash metal developed as a backlash against both the conservatism of the Reagan era and the much more moderate, pop-influenced and widely accessible heavy metal subgenre of glam metal which also developed concurrently in the 1980s. The early thrash metal movement revolved around independent record labels, including Megaforce, Metal Blade, Combat and Noise, and the underground tape trading industry in both Europe and North America. The genre was commercially successful during the mid-to-late 1980s and early 1990s, with the "Big Four" of thrash metal – Metallica, Slayer, Megadeth and Anthrax – being joined by Exodus, Overkill, Testament and Sepultura, as well as the "Big Four" of German thrash metal: Kreator, Destruction, Sodom and Tankard. Some of those bands are often credited for helping create, develop and popularize the genre. Wikipedia