following the regulative principle of worship, many reformed churches adopted the doctrine of exclusive psalmody: every hymn sung in worship must be an actual translation of a psalm or some other biblical passage. some reformed churches, especially the calvinists, rejected the use of instrumental music and organs in church, preferring to sing all of the music "a cappella". even today, the reformed presbyterian church of north america, the free presbyterian church of scotland, and other reformed churches of the scottish tradition maintain this practice.