Latest / The Copywriting Podcast with Fexingo: Sales Letters, Headlines, and Persuasive Writing / How a Single Underline Boosted Response by 47 Percent
Transcript
- Lucas: Today we are talking about a single underline. One line. And how it boosted direct mail response by 47 percent. Luna: A line? Like, a literal line under some text? Lucas: Exactly. A typesetting element so simple most of us don't think twice about it. But in the 1970s, a copywriter named Bill Jayme — he was the guy behind some of the most famous subscription letters for magazines like Psychology Today and Time — ran a test where the only variable was underlining the core promise in the body copy. Luna: And it lifted response by almost half. Lucas: Forty-seven percent, to be precise. Now, you might think, 'Sure, underlining draws attention, but that much?' The reason it worked so well is tied to how we read — or rather, how we scan. Eye-tracking studies show that when people look at a block of text, their gaze doesn't move in a smooth left-to-right march. It jumps. And things that are visually distinct — like an underline — act as anchors. They literally stop the eye and force a longer dwell time. Luna: So it's not about the underline itself being persuasive. It's that it makes the reader spend more time with the most persuasive sentence. Lucas: Right. And Jayme was a master of testing these small variables. He knew that in a direct mail package, the reader's attention is fragile. You have maybe a few seconds before they decide 'junk mail' and toss it. So anything that can buy you a fraction of a second more on your core argument matters. In his test, the underlined version of the letter pulled a 2.8 percent response rate against a control at 1.9 percent. That's the difference between a campaign that breaks even and one that prints money. Luna: And this was before computers made underlining trivial. In the 70s, it meant a typesetter literally inserting a rule under the line. It cost more. So Jayme had to justify that expense to the client. Lucas: Yeah, and that's the other interesting part: he had to argue for a production cost increase based on a hunch. But he had data from previous tests — smaller lifts, similar tactics — and he convinced them to run the split. That 47 percent lift became a case study that copywriters still reference. And it raises a bigger question: how many of us, today, are leaving similar easy gains on the table because we think formatting is trivial? Luna: Exactly. We obsess over word choice, headlines, offers — but the visual hierarchy of the copy itself gets ignored. And speaking of which, if these kinds of marketing conversations have sparked something you've actually used in your own work, it's worth mentioning that listener support is what keeps this show ad-free and independent. Lucas: Yeah. A few dollars a month genuinely makes a difference — buy me a coffee dot com slash fexingo, if you've gotten value from these episodes. It helps us keep digging up stuff like this 47-percent-underline story instead of running ad breaks. Luna: And we appreciate it. Anyway, back to the underline. So you've got this proven technique. How does it translate to a world of web copy, emails, landing pages? Lucas: Great question. On the web, underlining is mostly reserved for links — and that's a problem. Because users have been trained that underlined text means 'clickable.' If you underline a non-link sentence in a body of text, you risk confusing people. But the principle still applies: you need some form of visual emphasis. Bold, italics, color, even a different font weight or size. The key is to pick one element — the single most important sentence — and give it a distinct visual treatment. Luna: One sentence. Not three. Not the whole paragraph. Lucas: Exactly. Jayme underlined only one line in that letter: the promise. 'This is the most important message you will read this year.' Something like that. He didn't underline the headline, the subhead, or the guarantee. Just that one core promise. The reason is that if you emphasize too many things, you emphasize nothing. It's like using all caps — readers tune it out. But a single, deliberate highlight acts like a signal: 'Stop here. This is the point.' Luna: It's almost like a visual punchline. You lead the reader through the argument, and then you underline the payoff. Lucas: Exactly. And there's a second psychological layer: underlining mimics the act of a reader marking a book. When you see an underlined passage, it subconsciously feels like someone else has already deemed it important. It's a form of social proof embedded in the typography. That's why it worked so well in direct mail — it felt like the letter had been pre-highlighted for the reader by a trusted editor. Luna: That's a powerful insight. So if I'm writing an email today, I could test bolding the single most important sentence — maybe the main benefit or the call to action — and see if it lifts click-throughs. Lucas: Absolutely. And that's the beauty of this: it's a cheap test. You don't need a big budget. You just need a control version and a version where one sentence gets bolded. Run it for a thousand subscribers. If you see a lift, you've got a new baseline. If not, you learn something about your audience's visual preferences. The cost is zero. Luna: Are there any pitfalls? Like, what if you underline the wrong thing? Lucas: You can definitely emphasize the wrong thing. If you underline a weak sentence — a statement that doesn't carry the weight of the offer — you're basically shining a spotlight on a dud. The reader stops, reads it, thinks 'so what?' and then leaves. So you have to be ruthless. The underlined sentence must be the strongest, most benefit-driven, most emotionally resonant line in the entire piece. If you're not sure which one that is, ask three colleagues to read the copy and circle the most compelling sentence. Then test that one. Luna: That's a good rule. And it reminds me of something David Ogilvy said: 'The headline is the most important element.' But here, Jayme is saying the body copy's promise is worth highlighting. Lucas: Right. And it's not a contradiction. Ogilvy was talking about getting attention. Jayme was talking about persuasion after the attention is captured. The headline gets the reader in the door. The underline keeps them there long enough to absorb the main argument. They work together. In fact, in Jayme's letter, the headline did its job — it probably got a 20 to 30 percent readership. But the underline increased the conversion rate among those readers. So it's a sequential optimization. Luna: Makes sense. So for our listeners — if you're sitting down to write a sales page or a fundraising appeal today, what's your one takeaway? Lucas: Pick the single most important promise in your copy. Not the headline, not the call to action — the core benefit that makes someone say 'I want that.' And then give it a visual distinction. In an email, bold it. In a landing page, maybe use a different color or a larger font. In a letter, underline it. And then test it against a version without that emphasis. You might be surprised by the lift. Jayme's 47 percent is an outlier, but even a 10 percent improvement is worth having. Luna: And if you're worried about underlining on the web, just remember: the principle is visual hierarchy, not the line itself. Find your one anchor sentence. Lucas: Exactly. And we'll put a link to a classic Jayme letter in the show notes so you can see it in action. For now, that's the power of a single underline. Next time, we're going to look at what happens when you remove something from your copy — and why a blank space can sometimes be the most persuasive element of all.