Latest / The Copywriting Podcast with Fexingo: Sales Letters, Headlines, and Persuasive Writing / How Sentence Length Controls Reading Momentum
Transcript
- Lucas: There's this test from the 1930s that still haunts me. John Caples — the guy who wrote the ad 'They Laughed When I Sat Down at the Piano' — ran a split test on a mail-order course. Same offer, same headline, same guarantee. The only difference was sentence length. Luna: Just sentence length? Not the words themselves? Lucas: Right. So one version had sentences that averaged about 22 words. The other version alternated — some sentences were 35 words, some were 5 words. The alternating version pulled 18 percent more orders. Luna: Eighteen percent just from varying rhythm? That's wild. Lucas: Yeah, and here's what I love about it — Caples wasn't testing 'short sentences are better.' He was testing contrast. Long sentences create a kind of forward momentum, a build-up. Then a short sentence lands like a punch. It breaks the pattern, forces the reader to slow down and absorb. Luna: So it's not about simplicity, it's about surprise? Lucas: Exactly. If every sentence is short, the copy feels choppy, almost like a list. If every sentence is long, readers get fatigued — they start skimming. But alternating creates a rhythm that matches how we actually think. We have these long, winding thoughts, and then we sum them up in a snap. Luna: I remember reading that Caples was a huge believer in testing the 'invisible' parts of copy — not just headlines and offers, but the actual flow. Lucas: He called it 'invisible selling' or something similar. And it works because readers don't notice the sentence length consciously, but they feel it. There's a physiological response — your eye muscles literally move differently across a short sentence versus a long one. Luna: So how do you apply this today? I mean, we're not writing 1930s mail-order ads. Lucas: We absolutely are, just on screens. So take a landing page — the hero section has maybe 3 to 4 lines of copy. If you write two long sentences and then a short one, the short one almost always gets the click. I've tested this with clients. Luna: I did a version recently for a software onboarding page. The original was very uniform — all sentences around 14 words. I rewrote the first paragraph to start with a 28-word sentence, then a 6-word sentence. Click-through on the CTA went up 12 percent. Lucas: That's exactly the pattern. And it works because the long sentence builds context, creates a little tension or curiosity. Then the short sentence tells you what to do. 'Click here to start.' 'Request your demo now.' It feels like a resolution. Luna: But there's a risk, right? If you overdo the short fragments, it sounds like you're writing for a bad action movie. Lucas: Yeah, you don't want to sound like a parody of a direct-response writer. The key is deliberate variation. You need a reason for the short sentence — it should land on the most important point. If every sentence is a punch, no punch lands. Luna: So what's the ideal ratio? I've heard people say 'vary sentence length by at least 3 to 1' or something. Lucas: I don't think there's a magic ratio. But I'd say if you look at a paragraph and all your sentences fall within a 5-word range — say, 10 to 15 words — you've got a problem. You want some that are 20-plus, some that are under 8. And you want the short ones to be the key action step or the core insight. Luna: Let's talk about Caples' actual copy. Do you have an example? Lucas: Yeah, one of his classic ads for a writing course opens with: 'How often have you wished you could write better? Perhaps you have a book in you — or an article, a story, a letter that would win you promotion. But somehow the words won't come. You sit and stare at a blank page.' So the first sentence is 10 words, second is 21, third is 4 words, fourth is 6. That rhythm pulls you in. Luna: I can feel it. The 'But somehow the words won't come' — that short sentence — it's like a sigh. It mirrors the frustration. Lucas: Exactly. And notice the long sentence builds the dream: 'a book in you, an article, a story, a letter that would win you promotion.' Then the short sentence undercuts it with reality. That contrast is what makes the reader go 'yes, that's me.' Luna: And then the next short sentence 'You sit and stare at a blank page' — that's the pain point. It's almost cinematic. Lucas: Right. So the principle is: use long sentences to set the scene or build desire. Use short sentences to deliver the emotional punch or the call to action. And vary them unpredictably so the reader's brain stays engaged. Luna: I've also seen this applied in email subject lines. The most effective ones often have a short burst after a longer preview text. Lucas: Totally. The preview text is your long sentence — it sets up context. Then the subject line is your short sentence — the punch. 'Your free trial is ending. Don't lose your work.' The short subject line after a longer preview creates urgency. Luna: What about for social media? Short-form platforms like Twitter or Threads? Lucas: There, the constraint is already short. But you can still vary within a thread. Start with a long opening sentence — 30 characters — then a punchy one-liner. Or break a long thought into a series of short sentences for impact. The principle scales down. Luna: Let me ask you this — is there ever a situation where uniform sentence length is better? Lucas: Maybe in very technical writing where clarity trumps persuasion. Or in instructions where you want every step to feel equally important. But even then, a little variation can guide the reader's attention. 'Open the file. Then click Save As. Wait — before you do that, check the format.' That short wait — before you do that — it signals a caution. Luna: So it's about using length as a signal. Long = here's the context, short = here's the thing to remember. Lucas: Exactly. And the beautiful thing is, it's free. You don't need a bigger budget or a better offer. You just need to read your copy aloud and hear where the rhythm falls flat. Luna: That's the test I always recommend: read it out loud. If it sounds like a robot, your sentence lengths are too uniform. Lucas: If these marketing conversations have sparked something you've actually used — maybe you tried varying sentence length after hearing about Caples — that's exactly the kind of thing that keeps this show ad-free. Listener support is what lets us keep digging into these tests without selling anything. If you want to help, it's buy me a coffee dot com slash fexingo. Luna: Yeah, that's a huge part of why we can do episodes like this — because people chip in a few bucks. It honestly makes a difference. Lucas: So back to the rhythm — I want to give you a concrete exercise. Take a piece of copy you've already written. Highlight every sentence. Count the words. Then rearrange the paragraph so the long and short sentences alternate. Read it aloud. You'll almost always feel the improvement. Luna: I'm going to try that on a current project. I think I've been too uniform without realizing it. Lucas: One more tip: when you write a long sentence, end it with a short one that summarizes or contrasts. That's the Caples pattern. 'You've tried everything — the fancy templates, the clever headlines, the A/B tests that went nowhere. But this is different.' The short sentence after that long list — that's the hook. Luna: And the opposite also works, right? Short sentence to grab attention, then long sentence to explain. Lucas: Absolutely. 'Stop writing boring copy. Here's why most ads fail: they use the same sentence length over and over until the reader's brain checks out.' That's a short opener, then a long sentence that delivers the reasoning. Luna: So it's about variety, not about any one length being correct. Lucas: That's the whole game. And the best part is, you don't need a test for every sentence. You just need to develop an ear for it. After a while, you'll hear the monotony in other people's copy and your own. Luna: We should do a follow-up on Caples' actual testing methodology. He was such a meticulous tester. Lucas: That's a great idea — maybe dive into his 'scientific advertising' approach. For now, try the sentence length exercise. I'd love to hear if it lifts your numbers.