Heavy metal is a loud, aggressive style of rock music. The bands who play heavy-metal music usually have one or two guitars, a bass guitar and drums. In some bands, electronic keyboards, organs, or other instruments are used. Heavy metal songs are loud and powerful-sounding, and have strong rhythms that are repeated. There are many different types of heavy metal, some of which are listed below. Common features of heavy metal include slackened (downtuned) instruments and unusual key changes and time signatures (metre).