The organ is one of the oldest keyboard musical instruments, sometimes dated back to ancient Greece. It spread during the middle ages and took a prominent place as an instrument in Christian liturgical music. Eventually it took a place as a concert instrument, and a rich literature for organ music exists, with most famous composers having written at least some music for it. Most of the times, the term 'organ' applied in Western art music context, refers to the standard pipe organ that can be found in churches, and theaters, or to chamber organs. However, other forms of organ exists besides the pipe organ: there are mechanical organs, non-piped organs (which use reeds), and electronic organs (which create sound by electronic means and reproduces it through loudspeakers).