festive song, generally religious but not necessarily connected with church worship, often having a popular character. Today the carol is represented almost exclusively by the Christmas carol, the Advent carol, and to a lesser extent by the Easter carol. The tradition of Christmas carols goes back as far as the 13th century, although carols were originally communal songs sung during celebrations like harvest tide as well as Christmas. It was only in the late 18th and 19th centuries that carols began to be sung in church, and to be specifically associated with Christmas. Traditionally, carols have often been based on medieval chord progressions, and it is this that gives them their characteristic sound. Some carols like "Personent hodie" and "Angels from the Realms of Glory" can be traced directly back to the Middle Ages, and are among the oldest musical compositions still regularly sung. Carols suffered a decline in popularity after the Reformation in the countries where Protestant churches gained prominence (although well-known Reformers like Martin Luther authored carols and encouraged their use in worship), but survived in rural communities until the revival of interest in carols in the 19th century. The first appearance in print of "God Rest Ye Merry, Gentlemen", "The First Noel", "I Saw Three Ships" and "Hark the Herald Angels Sing" was in Christmas Carols Ancient and Modern (1833) by William Sandys. Composers like Arthur Sullivan helped to repopularize the carol, and it is this period that gave rise to such favorites as "Good King Wenceslas" and "It Came Upon the Midnight Clear", a New England carol written by Edmund H. Sears and Richard S. Willis. Christian hymnody Thomas Aquinas, in the introduction to his commentary on the Psalms, defined the Christian hymn thus: "Hymnus est laus Dei cum cantico; canticum autem exultatio mentis de aeternis habita, prorumpens in vocem." ("A hymn is the praise of God with song; a song is the exultation of the mind dwelling on eternal things, bursting forth in the voice.") The earliest Christian hymns are mentioned round about the year 64 by Saint Paul in his letters. The Greek hymn, Hail Gladdening Light was mentioned by Saint Basil around 370. Latin hymns appear at around the same time, influenced by Saint Ambrose of Milan. Early Christian hymns are known as canticles and are often based on Biblical passages other than the psalms; they are still used in Catholic, Lutheran, Anglican