Technical death metal (also known as tech-death, progressive death metal, or prog-death) is a musical subgenre of death metal that focuses on complex rhythms, riffs and song structures. Technical experimentation in death metal began in the late 1980s and early 1990s by bands such as Death, Atheist and Cynic. In 1990, Nocturnus released their debut album, The Key, which was followed by Sarcófago's third album, The Laws of Scourge, featuring a change in their musical style from black metal/thrash metal to technical death metal. Atheist's second album, Unquestionable Presence; Pestilence's third album, Testimony of the Ancients; and Death's fourth album, Human, were all released the very next year. Human and later Death albums have proven especially influential on later 1990s technical death