The term 'Christmas Carol' describes a song or hymn with Christmas-related lyrics, or traditionally sung during the holiday season. Though the first Nativity hymns may be traced to the 4th century Rome, they were mainly used as a vehicle for statement of theological doctrine. Modern Christmas Carols are sometimes based on medieval Christmas songs (in fact some are among the oldest musical compositions still regularly played), which sometimes gives them a distinct modal sound. Modern carols continue to be written too, to great success.