Hymns are to praise God, which is an important part of Christian religious ceremonies. These songs praising God were originally accompanied by musical instruments. The musical instruments used are: harp, harp, cymbal, horn, etc. Christian churches in the Middle Ages were private. Except for organ accidents, all musical instruments are exquisite and practical. Only "plainsong", which is given away by a single voice freely, is a legal church song. There are four schools of plainsong (also known as hymns) in western churches: the song of Ambrose named after Bishop St. Ambrose of Milan, Gaul plainsong of French church, Mosala Bouplainsong of Spanish church and Gregory plainsong named after Pope Gregory I. After the 8th century, Gregory plainsong became the highest standard of western church songs.