Hark The Herald Angels Sing is one of the most popular Christmas carols, but you may be surprised to find out the version that's been getting stuck in your head all your life isn't the original.

We probably don't need to tell you how the classic hymn starts, but here are the first two lines anyway:

Hark! The herald angels sing "Glory to the newborn king"

Now, here's the original, published in 1739 as simply Hymn for Christmas-Day:

Hark how all the Welkin rings "Glory to the King of Kings"

That sounds less like a carol and more like a half-remembered ditty from the Second Age of Middle Earth that you might find nostalgic Elves singing in Rivendell.

But "welkin" has nothing to do with Tolkien: the Oxford English Dictionary (OED) gives a range of definitions — "the apparent arch or vault of heaven overhead", "the celestial regions", "the upper atmosphere", etc — but basically, it's referring to the heavens.

This is how the hymn appeared in John and Charles Wesley's collection Hymns and Sacred Poems. ( Internet Archive )

Don't worry if it went over your head, the word had already fallen out of popular usage centuries before it was used poetically by Charles Wesley, one of the founders of Methodism with his brother John.

The change was made in 1753 by George Whitefield, their friend and fellow pioneering evangelical of the era, who also apparently had an ear for a banger (perhaps not surprisingly, given his massive fame and ability to draw a crowd has been likened to that of a rock star).

Whitefield made other changes too, but that was the one that stuck. The version we sing today came from a collection of hymns from 1760 (the compiler of which included a line of his own: With th' angelic host proclaim, "Christ is born in Bethlehem").

It's hard to imagine the carol taking off without Whitefield's rather crucial contribution. Indeed, there's a good chance it would have otherwise ended up like many of Charles Wesley's thousands of hymns — somewhat obscure.

But just as important as the rejigged opening lines is the music itself which, again, we have someone else to thank for — and which was actually composed more than 100 years after the hymn was first published.

In one early hymn book, from 1749, the song was set to Salisbury, the same tune used for another great Wesleyan hymn, Christ The Lord Is Risen Today:

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According to the Oxford Companion to Music, it was English organist WH Cummings who, in 1856, first set the words to the melody of the second number of German composer Felix Mendelssohn's Festgesang, which was composed in 1840.

That's the tune you know and love (or hate, if you're a grinch, that is).

Cait Miller, writing at the Library of Congress, says Cummings deserves more credit:

"Though his name is never printed alongside Mendelssohn nor Wesley, Cummings triumphs as the hero of this tale, fusing together the work of three men [Wesley, Whitefield, and Mendelssohn] who never could have foreseen such an enduring product."

Mendelssohn, dead by this point, might not have been particularly happy about this.

He wrote Festgesang to mark the anniversary of the invention of the Gutenberg printing press.

Mendelssohn thought his tune needed a "national and merry subject". ( Supplied )

According to Andrew Gant, writing in his 2014 book, Christmas Carols, the composer had expressly stated the piece would "never do to sacred words" and instead needed to be paired with a "national and merry subject".

As for how Charles Wesley would have felt, we don't know for sure. But given his love for the word "hark", we can assume he was glad it didn't get the chop at least. (If you're wondering what it means, it's basically just "listen".)

In general, though, his brother John wasn't a fan of people messing around with their lyrics.

Here's what he wrote in his Collection of Hymns for the Use of the People Called Methodists (1780):

"Many gentlemen have done my brother and me [though without naming us] the honour to reprint many of our hymns. "Now they are perfectly welcome so to do, provided they print them just as they are. But I desire they would not attempt to mend them; for they really are not able. None of them is able to mend either the sense or the verse. "Therefore, I must beg of them one of these two favours: either to let them stand just as they are, to take them for better for worse; or to add the true reading in the margin, or at the bottom of the page; that we may no longer be accountable either for the nonsense or for the doggerel of other men."

This wasn't just a case of John not wanting to be held responsible for someone else's dodgy rhymes. He didn't want the theology changed either.

Just to make your head spin a bit, here's what Hark The Herald Angels Sing/Hark How All The Welkin Rings sounds like with the original words and an alternative arrangement:

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Trippy…

Of course, Hark The Herald Angels Sing isn't the only Christmas carol to come from the brave old world of an intellectual property free-for-all. It's one of the many that, in Gant's words, have been gathered together like outcasts.

In his words, "Why do we sing carols in the versions we do? Because we just do."