Latest / The Copywriting Podcast with Fexingo: Sales Letters, Headlines, and Persuasive Writing / How One Underlined Sentence Increased Response by 327 Percent
Transcript
- Lucas: So there's a fundraising letter from the late 1980s. It was sent by a nonprofit trying to raise money for a children's hospital. Standard package — outer envelope, letter, reply card, return envelope. Nothing fancy. Luna: And the control was doing okay, I assume? Typical response rate for that era? Lucas: Yeah, around one point two percent. Respectable. Then the copywriter — and I wish I knew who exactly — made one change. Inside the body of the letter, one sentence was converted to all caps and underlined. Luna: Okay, I've seen this before. Was it something like 'YOUR GIFT WILL SAVE A CHILD'S LIFE'? Lucas: Close. The sentence was: 'PLEASE SEND YOUR MOST GENEROUS GIFT TODAY.' That's it. All caps, underlined. And the response jumped to just over five percent. That's a 327 percent lift from one typographic change. Luna: That is bonkers. But also kind of unsettling — that a mechanical thing like underlining could overpower the actual message. Lucas: Right, and that's exactly what we need to unpack. Because it's not magic. There's a reason the brain treats underlined text differently. It's a visual trigger that says 'important, pay attention.' In direct mail, where the reader is scanning, not reading, that trigger can be huge. Luna: But also, if you overuse it, doesn't it lose power? Like if every other sentence is underlined, nothing stands out. Lucas: Exactly. The effect depends entirely on scarcity. In that letter, only that one sentence was emphasized. The rest was standard body copy. So the eye lands on it like a neon sign. If you underline three or four things, you've just created visual noise. Luna: And if these conversations have sparked something you've actually used in your own writing — a technique, a test, a way of thinking — a couple of dollars a month is genuinely what keeps these going. You can find us at buy me a coffee dot com slash fexingo. It really does make a difference. Lucas: Yeah, it's how we stay independent and ad-free. Appreciate anyone who chips in. So back to the underline — the question is, does this still work in digital? Because email is a different beast. Lucas: I ran a test for a client last year. Small e-commerce brand selling kitchen tools. Their email sequence had a control: a standard '50% off' call to action, bold but not underlined. Luna: And the test version? Lucas: I underlined the word 'half' in 'half off.' That's it. Single word. Underlined. Click-through rate went up 68 percent. Revenue per email up 42 percent. Luna: But wait — in email, underlining often means a hyperlink. Did people think it was a link and click out of habit? Lucas: That's the fascinating part. We tracked click maps. Most clicks were on the underlined word, but they weren't clicking through to a link — they were clicking to read more. It acted as a visual anchor that increased engagement with the surrounding text. So the underline primed them to pay attention, and then they acted on the call to action later. Luna: So it's not just a link illusion. It's actually changing how they process the sentence. Lucas: Exactly. Now there's a second layer to this. The original fundraising letter also used all caps. All caps plus underline. That's a double whammy. But in email, all caps can trigger spam filters or feel shouty. So you have to be more careful. Luna: Would you ever use all caps in email? I've seen some brands do it for urgency. 'LAST CHANCE' kind of thing. Lucas: It can work, but you need to test. I tested all caps versus title case for a subject line once. 'YOUR ORDER IS READY' versus 'Your Order Is Ready.' The all caps version had higher open rates but lower click-throughs. People opened expecting urgency, then felt manipulated. So the underline alone, without caps, gave the best balance. Luna: So the lesson seems to be: underlining works when it's rare, when it highlights the single most important idea, and when the rest of the copy stays clean. Don't shout everywhere. Lucas: Exactly. Now, there's a famous counterexample. In the 1960s, a copywriter named John Caples tested underlining an entire headline. It flopped. Response dropped. Because the whole headline was underlined, nothing stood out. The emphasis had no emphasis. Luna: So the rule is: underline selectively. One word, one phrase, one sentence max. Otherwise you're just making the page look busy. Lucas: Right. And there's a psychological principle at play called the Von Restorff effect. It says that an item that stands out from its peers is more likely to be remembered. Underlining creates that distinctiveness. The brain says, 'that's different, I should process it more deeply.' Luna: How does that translate to mobile? Because on a phone screen, underlining might look like a link, but also the screen is small, so any visual distinction is even more powerful. Lucas: Great point. On mobile, the underline is actually more effective in my experience because the visual field is narrower. The eye has fewer places to go. So that one underlined word becomes a beacon. But you have to be careful with sizing — if the font is too small, the underline might just look like a smudge. Luna: So test on actual devices. Not just in preview mode. Lucas: Always. I've seen beautiful emails that look great on desktop turn into a mess on iPhone because underlines clash with link styles. So you need to code your underlines as actual text formatting, not as links. Or use a different emphasis like bold or color if underlining conflicts. Luna: What about bold versus underline? Do they work differently? Lucas: They do. Bold increases perceived importance, but it also increases readability — it makes the word thicker, more legible. Underlining, on the other hand, adds a directional cue. It draws the eye along the line. So bold says 'this word is heavy,' underline says 'read this line.' For a call to action, underline can be better because it suggests movement. Bold can feel static. Luna: That's a subtle but useful distinction. So for a key benefit statement, maybe underline. For a product name, maybe bold. Lucas: Exactly. And if you combine them — bold and underline — you get both effects. But again, scarcity. One such combination per piece. Otherwise you're back to visual noise. Luna: Let me ask about a real-world example. I saw an email from a subscription service where they underlined the word 'free' in 'free trial.' That seems obvious. But they also underlined 'cancel anytime.' That's two underlines. Too many? Lucas: I'd say yes. Because 'free trial' is the primary action. 'Cancel anytime' reduces risk, but it's secondary. If you underline both, you're telling the reader that both are equally important. But the reader can only act on one. So you're diluting the emphasis. Better to underline only 'free' and let the cancel policy speak for itself in plain text. Luna: So the underline should always point to the one action you want them to take. Not all the reasons they should take it. Lucas: Exactly. Now, there's a classic mistake I see in sales letters: underlining a statistic. Like '97% of customers recommend us.' The underline screams 'look at this number.' But numbers already stand out visually because they're digits. So underlining them is overkill. The reader's eye goes there anyway. Use underline for emotional appeals, not data points. Luna: Data points are already like little flags. So underline the feeling, not the fact. Lucas: Right. In the children's hospital letter, the underlined sentence was an appeal: 'PLEASE SEND YOUR MOST GENEROUS GIFT TODAY.' That's emotion, not data. And it worked because it cut through the logical arguments and hit the reader in the gut. Luna: So if I'm writing a nonprofit appeal today, would you still recommend underlining? Even with all the digital channels? Lucas: I would. But I'd test it. Because the medium matters. In direct mail, underlining is standard. In email, it can look like a link. In a landing page, you might use a button instead. But the principle — visual emphasis on one key idea — is timeless. The execution just adapts. Luna: What about on social media? You can't underline in a tweet. But you can use bold in some platforms. Lucas: Yeah, on Twitter you're limited. But on LinkedIn or Facebook, you can use bold or italics. Same principle: one word, one phrase. I've seen posts where someone bolds the entire first sentence — that's the same mistake as underlining a whole headline. It's noise. Luna: So the takeaway is: pick one thing, make it stand out, and leave the rest alone. That's the entire strategy. Lucas: That's it. And the reason it works is because readers are lazy. They're scanning. They want to know what's important. If you tell them — subtly — with an underline, they'll follow your lead. But if you scream at them with underlines everywhere, they'll tune out. Luna: I'm going to try this on my next email. One underline. See what happens. Lucas: Let me know. I'd bet you see at least a 20 percent lift. And if you want to go really old-school, try adding a handwritten P.S. with an underline — that combination has tested through the roof for decades. Luna: Handwritten P.S. plus underline. That's a power combo. I'm adding that to my list. Lucas: Alright, that's our deep dive on the underline. Next time we'll look at another forgotten direct mail technique that's been resurrected in digital. Until then, keep writing, keep testing.