Post-metal is a style of music that is rooted in heavy metal but explores approaches beyond the genre's conventions. It emerged in the 1990s through the work of bands such as Neurosis and Godflesh who transformed metal texture through experimental composition. Associated with and inspired by post-rock and post-hardcore, the genre employs the darkness and intensity of extreme metal but emphasizes atmosphere, emotion, and even "revelation", drawing on a wide range of sources including ambient, noise, psychedelic, progressive, and classical music to develop an expansive but introspective sound. Post-metal songs are typically long, with loose and layered structures that discard the verse–chorus form in favor of crescendos and repeating themes. The sound centres on guitars and drums; any vocals are usually screamed or growled and resemble an additional instrument.