The organ repertoire consists of music written for the organ. Because it is one of the olest musical instruments in existence, written organ repertoire spans a time period almost as long as that of written music itself. The organ's solo repertoire is among the largest for any musical instrument. Because the organ was found almost exclusively in the western churches from the Middle Ages until the emergence in the Romantic era of large concert hall instruments, a significant portion of organ repertoire is sacred in nature. Thus, most organ music comes from Western Europe and North America; because the eastern religions (such as the Eastern Orthodox Church) did not use any instruments in their worship. Owing to the age of the instrument and its frequent use in Western worship, the organ has one of the most varied repertoires of any instrument.