But why should this be so? What accounts for the overwhelming popularity of the Christmas carol? For many of us, certainly, carols are part of childhood memories. But the likeliest reason for their prominence is that they are a vibrant, colorful strand of folk music with deep cultural roots, both Christian and pagan. They instantly evoke that great synthesis of midwinter holiday customs. Surging hymns, like Adeste fideles, are inextricably linked with carols, so much so that we tend to group the two types together. But it is the carols that bring us close to timeless seasonal rhythms that, in our frenetic, half-baked ways, we seek to honor, as our ancestors did.

Christmas, of course, was deliberately placed close to the winter solstice. The Christian message of renewal contained in Jesus’ birth is symbolized by the lengthening hours of sunlight and the promise of spring that follows the shortest days of the year. And so it coincides with all sorts of non-Christian midwinter observances. Similarly, the customs and music of Christmas can be parsed into Christian and non-Christian segments that work in tandem.

Look no farther than the partridge in a pear tree for an example, as Leigh Grant explains in The Twelve Days of Christmas: A Celebration and a History. In the 18th century, when the song came into being, it was customary to pour wassail, a cider libation in a wooden wassail bowl, around a fruit tree, in a reference to an ancient fertility rite. The party would also fire shots into the tree’s branches to ward off evil spirits.

In a related superstition, if a girl walked backward three times around a pear tree on Christmas morning, she could glimpse the face of her future husband. Fruit, of course, has always been symbolic of fertility, as have certain animals, such as the male partridge, reputed in folklore to be promiscuous and, writes Grant, “a lusty suitor.”

Carols like God Rest Ye Merry were — and still are — popular songs. As opposed to hymns, the music of carols has swing, and the rhythmic vigor of tapping feet and beating drums. And that’s where the genre originated. The word, with roots in Greek, choraules (choral song), referred to round dances, performed with song. These were common at all levels of medieval society and were important parts of festive occasions.

The Spanish carol Fum, fum, fum, probably written between the 16th and 17th centuries, is traditionally sung after midnight Mass as people promenade on the streets with torches, tambourines, and guitars. The recurring “Fum, fum, fum!” is thought to be an imitation of an instrument — perhaps the strumming of a guitar or the tapping of a drum. The “fum” is similar to the sound of the Andalusian zambomba, a friction drum made from a clay vessel covered with animal skin — usually goat — or some sort of thin material. In the center of the covering a long cane is secured which produces a deep sound when rubbed with the hand. In Andalusia, the zambomba gives its name to a Christmas gathering, in which flamenco carols — such as Fum, fum, fum — are sung and danced. As much as any tune, it suggests the connection between carols and dance, with its vigorous rhythm and the repetitions of its head motive, often accompanied by clapping.

The Welsh carol Deck the Halls (Nos galan, meaning “New Year’s Eve”), belongs to a competitive song tradition called canu penillion, which is still thriving in Wales. According to The Shorter New Oxford Book of Carols, “merrymakers would dance in a ring around a harpist, extemporizing verses in turn and dropping out when invention failed. The harp originally played the ‘answering’ bars (3-4, etc.), but nonsense syllables came to be substituted as harpers became less common.”

The idea of a carol as a devotional song arose gradually. The many carols we have from the 15th century are distinguished by their form, usually with a refrain alternating with uniform verses. Of course, carols were not exclusively meant for Christmas, originally, as The Agincourt Carol shows. It celebrated a major military victory for the English in the Hundred Years' War. By the 16th century, however, carols were extremely variable in form and style. Interestingly, as its music became more diverse, the subject matter became more focused on the Christmas holiday.

Popular tunes are flexible, and authors of carols often have borrowed melodies from older songs. For Good King Wenceslas, honoring the 10th century Duke of Bohemia, the 19th century English clergyman John Mason Neale adapted a 13th century spring carol, Now the Time of Flowers Comes, first published in Sweden in 1582. Bring a Torch Jeannette Isabella, a French Provençal carol published in the 16th century, takes its melody from courtly dance music. What Child Is This, written by William Chatterton Dix in 1865, was arranged, probably by John Stainer, to the famous 16th century melody of Greensleeves. That melody is itself the source of much conjecture, but like most ballads, its origins are lost to us. The oldest extant print copy dates from 1580.

Since they are relatively far from the written tradition, it is unusual to find an old carol with an “original” tune. In fact, some carols have more than one tune associated with them, as is common in folk music. God Rest Ye Merry has three, the one we know being the most often associated with it. Like many other folk songs, God Rest Ye Merry was not printed until it was collected for an anthology. In this case, William Sandys, a solicitor and fellow of the Society of Antiquities, took down the tune for his landmark collection, Christmas Carols Ancient and Modern (1833). Among the other carols that were printed for the first time in Sandys’ book were The First Nowell and I Saw Three Ships.

The tunes for Christmas carols have also been known to migrate into classical music. Chopin used the tune of Lulajze Jesuniu (Lullabye Jesus) for his Scherzo in B Minor. None other than J.S. Bach wrote a complex arrangement of In dulci jubilo, by the 14th century Dominican monk Heinrich Suso, a carol we know as Good Christian Men, Rejoice, in J.M. Neale’s translation.

The Italian tradition of lullabies to Jesus, evocative of his birth in a manger, established itself as a tradition in classical music. The gently rocking rhythms of these songs, generally in compound meters, sometimes employing a “siciliano” dance rhythm (a moderate 6/8 or 12/8, with dotted rhythms), became established in the classical repertory in the music of the Neapolitan Alessandro Scarlatti. Under the generic title “pastorale,” (“pastoral,” referring to Jesus’ birth in a manger), they became a necessary part of any Christmas-themed piece. Pastorales show up in Arcangelo Corelli’s Christmas Concerto, Bach’s Christmas Oratorio, and the first part of Handel’s Messiah.

Of course, the popular source of the music of Christmas carols and their dancelike rhythms shouldn’t earn them scorn or imply a lack of piety or religious orthodoxy. The religious messages contained in the carols are the most important layer in their composition — it’s the thing that identifies them as Christmas carols. In fact, the music accords remarkably well with the imagery and spirit of the Christmas story, which is told from the point of view of shepherds.

At the other end of the spectrum, the reverent but operatically inclined O Holy Night (Minuit, Chr’tiens in the original French), composed in 1847 by Adolphe Adam (of Giselle fame), is more of an oddity than the usual popular song carol. Despite its status as a Christmas classic, the Roman Catholic Church originally disapproved of it, and a French bishop denounced its "lack of musical taste and total absence of the spirit of religion." That climactic high note must have sounded awfully suspicious to churchmen who were anxious to divorce religious music from the taint of the theater.

But the admonition came too late. Midnight Mass at a cathedral (the one liturgical event in the year deemed worthy of network television coverage), the street processions that take place around the world during the 12 Days of Christmas, the lights, the nativity story itself — all of it is theatrical and dramatic. And beyond the specifically Christian layer of the feast lies all the folkloric and ancient traditions that have such a magical resonance for us who, prodded by the pace of urban capitalism, rarely take the time to look into the night sky. As Dickens and Dr. Seuss proved, the carol is the glue, the musical symbol that connotes all of these things at once.

(Michael Zwiebach holds a Ph.D. in music history from UC Berkeley.)