Of all rock & roll's myriad forms, heavy metal is the most extreme in terms of volume, machismo, and theatricality. There are numerous stylistic variations on heavy metal's core sound, but they're all tied together by a reliance on loud, distorted guitars (usually playing repeated riffs) and simple, pounding rhythms. Heavy metal has been controversial nearly throughout its existence — critics traditionally dismissed the music as riddled with over-the-top adolescent theatrics, and conservative groups have often protested what they perceive as evil lyrical content.