In the middle of this year’s carol service you may find yourself ambushed by one of the following random thoughts.

a) “Good my page” (“Good King Wenceslas”). Why isn’t it just “my good page”? Oh, hang on, maybe that’s how it was in the original ... Czechoslovak?

b) “G-l-o-o-o-o-o-r-i-a” (“Ding Dong Merrily on High”). I can’t hold my breath much longer. Actually, I think I might be hyperventilating. I’m definitely feeling weird. I need to hang on to the pew in front.

c) “I Saw Three Ships”. But Bethlehem is landlocked! Isn’t it? If I keep my phone in my handbag, I can Google it during the next reading.

Carols inspire untidy thoughts because they are themselves a muddle. While they may appear in church in their Sunday best, they started as folk songs, bellowed out in fields and pubs as the year crept round to its darkest days. It was this antic energy that caught the ears of a successive wave of “improvers” – Benedictine monks, Lutheran preachers, Anglo-Catholic clergymen, sentimental antiquarians – who gussied up the peasant grammar, added a sprinkle of theology, and crammed old words into new tunes. Yet, despite presenting themselves to us in their smartest rig, carols still bear the marks of their ramshackle making. You can see it in all those things that don’t quite fit: oddly stressed syllables, the sudden swerve from vernacular to liturgy and, above all, something called melisma.

Don’t worry if you don’t know what melisma is. Andrew Gant is on hand to explain that it’s that weird elongation you get in carols like “Ding Dong Merrily” when Glo-o-o-o-ria goes on for ever. Or in “Good King Wenceslas”, when the page goes gathering winter f-u-u-e-l for no good reason. Gant used to be choirmaster at Her Majesty’s Chapel Royal and now lectures in music at Oxford, so his approach to our carols’ mongrel past is as far away from one of those novelty heritage Christmas guides as you can imagine. Instead, he provides us with a fluent but flinty digest of the modern scholarship concerning 22 of our favourite seasonal songs. With Gant as our guide, we travel the English and French folkways, hang over the west gallery (home of Georgian psalmody) and hover above the shoulder of countless Victorian clergymen as they scribble away in their studies. We even take a trip to America, where carols are often required to embrace the ticker-tape culture, zany costumes and all. “We Three Kings”, it turns out, was written in 1857 by a Pennsylvanian rector as a rousing finale to his seminary’s Christmas pageant.

The premise for Gant’s book is similar to that adopted by food writers a decade ago, in which a clever scholar would unpick a recipe for beef wellington and lay bare the history of the world. Here Gant is, then, on “O Come All Ye Faithful”: its recordable history starts when a devout Catholic from Leeds fled to the continent following the Protestant settlement of 1688 and consoled himself in Douai, northern France, by copying out “Adeste Fideles”. From here, the hymn – not quite a carol yet – set off in a dance around the world, picking up fabled makers along the way. The words were said to be by the king of Portugal or by a 13th-century saint, the music was perhaps by Handel or maybe it was a bit of comic-opera doggerel.

Then, in the 1940s, a Benedictine scholar suggested that the hymn was actually a coded call to arms for the 1745 rebellion of Bonnie Prince Charlie. “Fideles” are Catholic revolutionaries, “Bethlehem” is the true church in captivity and “Natum videte, regem angelorum” is a pun: Christ was born king of the angels, but the Young Pretender was born king of the English.

All of which is so subtle that it’s no wonder that the stolid Hanoverians, by now ensconced in place of the flighty Stuarts, never spotted the double entendres. “Adeste Fideles” was absorbed smoothly into the Georgian repertoire and the Catholic call to arms became a Protestant tub-thumper. First the words and then the music were changed to something more bracingly English. Where once the faithful minuetted into Bethlehem in 3/4 time, now they approached at a brisk 4/4 march.

Give or take the fact that it might be written in code, “O Come All Ye Faithful” is easy to follow. It takes us through the liturgical meaning of the nativity sequence, ending with that most thrillingly literal verse: “Yea Lord we greet thee/ Born this happy morning.” Most carols are much more of a jumble, having picked up all sorts of non sequiturs along the way. “I Saw Three Ships”, for instance, started in Aberdeen and travelled via Kent, hence the way Bethlehem has mysteriously acquired a coastline. In “The Holly and the Ivy” we leap awkwardly from the pagan evergreens to the distinctly churchy “playing of the merry organ”, which suggests that someone had a go at Christianising it along the way. What’s more, the line won’t fit the tune, no matter what metrical and musical contortions you try.

But when it came to creative fiddling, the Victorian Anglo-Catholics were the past masters. Determined to add a dash of medieval mystery to the brash beat of militant Protestantism, they invented an archaic language that was about as authentic as a pointy spire by George Gilbert Scott or a wimple from William Morris. “Good my page” could indeed just as easily be “my good page”, while in “The Angel Gabriel Came Down”, the line “for known a blessed mother thou shalt be” is simply a bit of Victorian whimsy about how people spoke in the olden days. “Ding Dong Merrily”, meanwhile, is a treasure chest of made-up gibberish: “swungen” and “sungen” are just about credible as Old English borrowings, but it’s difficult to imagine any self-respecting peasant singing “Ding Dong” in whatever century he was supposed to be capering.

All of which makes it doubly confusing to learn that, when a carol did occasionally appear in an authentic older form, it pleased no one. Charles Wesley’s original opening line for “Hark the Herald Angels Sing” was “Hark how all the welkin rings”. “Welkin”, a word known to Shakespeare, denotes the sky, the firmament, the whole infinite space in which herald angels are apt to hover. Yet when Ralph Vaughan Williams, that tireless restorer of ancient musical forms, tried to get people to harken to the welkin in the early 20th century, they were appalled. It sounded ridiculous, if not slightly disgusting, as if the congregation was being asked to attend to a rudely shaped vegetable.

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