Here are some things that happen when you go viral on Twitter for pointing out that the first two lines of Stephen Sondheim’s “The Ballad of Sweeney Todd” can be sung to the tune of Leonard Cohen’s “Hallelujah”:

Your notifications will blow up with hyperbolic expressions of anguish and hostility, Twitter’s preferred mode of praise. (“I hate this.” “This hurts me.” “This can’t be legal.” “Quick question, how dare you?” “A curse upon you.” “The mindfuck of this has given me a deeper appreciation for characters in Lovecraftian horror. It … should not be.”) The Classic FM website will run a story on you headlined “Someone is setting Sweeney Todd lyrics to the tune of ‘Hallelujah’ and it’s honestly fantastic,” misidentifying you as “a young writer from Connecticut, US.” Your mother will kvell over her viral daughter on Facebook and in a mass email to all her friends. You will wonder why this is all happening around this tweet, which is decidedly B material, while your A material languishes in obscurity.

Above all, though, you will be confronted by men who insist on being confidently, floridly wrong at you. I’m given to understand that this is common on Twitter in general, but up till this point, my anonymity and gender ambiguity had spared me. Once I went viral, though, the men-who-were-wrong came out in full force. One guy in particular—a partner at a law firm, according to his Twitter bio—retweeted me along with the enthusiastically incorrect remark, “Iambic pentameter FTW.” And with that, I realized why so many people were so disproportionately impressed by my Sweeney Todd/Hallelujah observation: a widespread misunderstanding of how meter works.

At the risk of giving away the secret to my success, I’d like to demystify meter for the good of the people.

The “Iambic pentameter FTW” reply guy was wrong on two counts. First of all, the lyrics in question are not in iambic pentameter, a poetic meter best described as “the one that goes da-DA da-DA da-DA da-DA da-DA” (that is to say: five iambs, or “da-DA”s, per line). And second of all, FTW, or “for the win”—a prepositional idiom that Wiktionary defines as “being the best; being great, awesome, amazing or spectacular; sure to succeed”—is not a phrase that remotely applies to iambic pentameter, the most overrated of all meters. It’s a perfectly serviceable meter, but it’s the Sweeney Todd/Hallelujah tweet of meters: with so many other good ones out there, why does that one get all the glory?

Because of school, of course. Most of us learn about iambic pentameter in the context of Shakespeare, a guy who loved himself some iambic pentameter, especially in his sonnets. His plays, however, don’t rely on it quite as much as your high school English teacher may have led you to believe. If it’s been a while, you might surprise yourself by cracking open a Shakespeare play and looking at how often his characters speak in unmetered prose; in fact, if you’re looking at Much Ado About Nothing or The Merry Wives of Windsor, you won’t find any metered verse at all. When Shakespeare’s characters do use verse, it’s not limited to iambic pentameter: Puck and his fellow fairy underlings in A Midsummer Night’s Dream speak largely in trochaic tetrameter (“If we shadows have offended / Think but this and all is mended”).

As a rule, Shakespeare reserves iambic pentameter exclusively for his aristocratic characters. That’s why A Midsummer Night’s Dream uses iambic pentameter for the dialogue of the Duke and Duchess of Athens, the King and Queen of the Fairies, and the four high-born lovers, while the working-class “rude mechanicals” speak entirely in prose. When you go to Hamlet and hear the title character speak his first line—“A little more than kin, and less than kind”—the iambic pentameter is Shakespeare’s way of signaling to you, just in case you missed it from context clues, that Hamlet is a prince.

Iambic pentameter also served a second, more practical function for the ever-practical Shakespeare: it made long speeches much easier for an actor to memorize. Onstage, an Elizabethan theater star could pass off his iambic pentameter soliloquies as spontaneous, off-the-cuff speech (“But soft! What light through yonder window breaks? It is the east, and Juliet is the sun!”) after having privately drilled them like military cadences (“But soft what light through yonder window breaks / It is the east and Juliet is the sun”). This trick still works, by the way, even with contemporary English: the comedian Jacqueline Novak admitted in a recent interview that part of her one-woman show, On Your Knees, is secretly written in iambic pentameter—not for the audience to notice, but just so she could memorize it quickly.

The idea behind this is that iambic pentameter, out of all poetic meters, most closely mimics the natural rhythms of casual spoken English. As far as I know, this is not an established linguistic fact, but there does seem to be something to it. Some years back, a friend and I went to see La Bête, a contemporary play written in iambic pentameter as a gimmick; afterward, on the subway home, we kept cracking up to find ourselves inadvertently speaking in iambic pentameter. (“I really liked the part with what’s-her-name, the princess, with the sparkles on her gown—” “You’re doing that on purpose!” “No! Are you?”)

But there’s a flip side to iambic pentameter’s conversational quality: it is a distinctly unmusical meter. You will never hear a song whose lyrics are in iambic pentameter. I just typed that sentence on a hunch, but then I Googled “iambic pentameter songs,” and the first result, a peculiar piece of clickbait called “Six of the Greatest Lyrics Sung in Iambic Pentameter,” proves my point: of the six highly obscure entries on that list, four of them are not in iambic pentameter at all, and the remaining two are isolated lines that fit the meter only by coincidence. It’s simply not well suited to music. If we wanted songs to sound indistinguishable from speech, all radio would be talk radio.

So, to reiterate, this Twitter man was mistaken: my Sweeney Todd/Hallelujah mash-up is not in iambic pentameter. I wasted no time informing him so. “Nope, it’s actually iambic tetrameter,” I admonished him in a public reply. “And more saliently it’s ballad meter.”

But with great viral fame comes great responsibility, and this is the point at which I must admit that I, too, was wrong. It’s not ballad meter, either.

In my defense, I had good reason to assume it was. First of all, the song is literally called “The Ballad of Sweeney Todd.” Second of all, ballad meter is the poetic meter that most famously lends itself to this type of comical lyric-swapping. The first such gag I ever encountered, as a child, was the axiom that every Emily Dickinson poem can be sung to the tune of “Yankee Doodle.” This turns out to be an exaggeration—not every Dickinson poem fits—but it works delightfully often; try it with “Because I could not stop for Death.” Better yet, try it to the tune of the theme song from Gilligan’s Island. Or, for something more thematically appropriate, swap in the words of Coleridge’s “The Rime of the Ancient Mariner.” Play around with mixing and matching the tunes and lyrics of “God Rest Ye Merry, Gentlemen,” “O Little Town of Bethlehem,” the theme song from Pokémon—or my very favorite:

The combinations seem endless. But you can’t do it with “The Ballad of Sweeney Todd,” and you can’t do it with “Hallelujah.” Or, rather, you can do it with the first line—“Attend the tale of Sweeney Todd” and “I’ve heard there was a sacred chord,” respectively—but it breaks down in the second. That’s because ballad meter is actually two different meters stuck together: each verse is a quatrain that alternates between iambic tetrameter and iambic trimeter. In layman’s terms, this means that each verse is four lines long; the first and third lines go “da-DA da-DA da-DA da-DA” (four iambs), while the second and fourth lines go “da-DA da-DA da-DA” (three iambs).

“The Ballad of Sweeney Todd” and “Hallelujah” don’t alternate like that: their first and second lines are both in iambic tetrameter. Pure iambic tetrameter is not as musically common as ballad meter, but it’s somewhat easier to write in, hence its irresistible comic potential. In fact, lyrical parodies of Leonard Cohen’s “Hallelujah” are already a thriving subgenre of social media joke. As tumblr user amatalefay pointed out in September 2017:

My first childhood encounter with a pure iambic-tetrameter lyric swap remains my favorite: Robert Frost’s poem “Stopping by Woods on a Snowy Evening” sung to the tango tune of “Hernando’s Hideaway” from The Pajama Game. If you’re not at that level of musical theater geekery, you can also swap in one of these more familiar tunes, as long as you stick to the first two lines:

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Feel free to try it with other poems in iambic tetrameter:

I think that I shall never see
A poem as lovely as a tree
But you don’t really care for music, do ya?

In Xanadu did Kubla Khan
A stately pleasure-dome decree
He shaved the faces of gentlemen
Who never thereafter were heard of again.

’Twas brillig, and the slithy toves
Did gyre and gimble in the wabe:
All mimsy were the borogroves, Jolene.

But—you may be wondering at this point—why does this trick work in only one direction? It’s all well and good to sing “The Ballad of Sweeney Todd” to the tune of “Hallelujah,” but why can’t you do it the other way around? For that matter, why does it work only when you limit yourself to the first two lines of the song? Why can’t you make the whole tune fit?

Generally speaking, the answer is boring: most song lyrics don’t fit any particular poetic meter. They don’t need to. On the page, a poet must rely on meter alone to convey rhythm and structure, but if you bring music into the mix, well, rhythm and structure are what music is. When song lyrics do fall into an identifiable poetic meter, the phenomenon is rare enough to be ripe for comedy, as we’ve just seen.

In the case of “The Ballad of Sweeney Todd,” though, there’s a bit more to it. Sondheim’s use of iambic tetrameter is no accident: Sweeney Todd is set in Victorian England, and “The Ballad of Sweeney Todd” deliberately models itself after Victorian poetry, which is heavily metered. The song’s verses do abandon their iambic tetrameter after the first two lines, but that doesn’t mean Sondheim abandons meter altogether. Let’s take a look at what he does instead:

Verse 1:

Attend the tale of Sweeney Todd
His skin was pale and his eye was odd
He shaved the faces of gentlemen
Who never thereafter were heard of again

Verse 2:

He kept a shop in London Town
Of fancy clients and good renown
And what if none of their souls were saved?
They went to their Maker impeccably shaved 

“Who never thereafter were heard of again.” “They went to their Maker impeccably shaved.” Da-DA da-da-DA da-da-DA da-da-DA. What we have here is another mash-up meter: half iambic tetrameter, half anapestic tetrameter.

I’ve taken several poetry classes in my life, and every time the teachers set out to explain anapestic tetrameter, they always use the same terrible example: Lord Byron’s “The Destruction of Sennacherib.” I find this choice baffling for three reasons. For one thing, who still reads Lord Byron? For another, it’s not at all clear to the modern reader that the word “Assyrian” is meant to be read as three syllables, rather than four, and this invariably results in a roomful of students awkwardly misreading it aloud—“The uh-SEER-ee-an CAME down like UH wolf on THUH fold”—until the wheels come off the wagon. Most importantly, though, American popular culture is full of much more familiar, much better examples of anapestic tetrameter:

’Twas the night before Christmas, when all through the house
Not a creature was stirring, not even a mouse;
The stockings were hung by the chimney with care
In hopes that St. Nicholas soon would be there.

Or better still:

“I know some good games we could play,” said the cat.
“I know some new tricks,” said the Cat in the Hat.
“A lot of good tricks. I will show them to you.
Your mother won’t mind it at all if I do.” 

Sondheim’s Sweeney Todd metric combination, however, is quite unusual. Very few poets alternate between iambic tetrameter couplets and anapestic tetrameter couplets. In fact, in all my research, I’ve come across this form only one other time—which is to say that there’s only one other lyric that can be sung to the tune of “The Ballad of Sweeney Todd.”

It gives me no pleasure to tell you what it is. In fact, you’re probably better off not knowing. Once you know it, you can never un-know it. Stop reading this right now. It’s too late for me, but you can still save yourself.

Well, don’t say I didn’t warn you.

Jellicle Cats come out tonight
Jellicle Cats come one, come all
The Jellicle Moon is shining bright
Jellicles come to the Jellicle Ball

Yes, it’s true: the title song from Sweeney Todd is metrically interchangeable with the title song from Cats.

I hereby relinquish my viral fame and accept my cancellation.

James Frankie Thomas is the author of “The Showrunner,” which received special mention in the 2013 Pushcart Prize Anthology. His writing has also appeared in The Toast, The Hairpin, and Vol. 1 Brooklyn. He holds an MFA in fiction from the Iowa Writers’ Workshop.