Preview text is the second subject line you never wrote

Every serious email marketer writes and rewrites their subject lines. Every serious email marketing course dedicates at least one module to them. Every A/B test that has ever produced a chart has been about subject lines. And meanwhile, right next to the subject line in every inbox on earth, sits an equally important second line that almost nobody writes on purpose. The preview text.

The preview text — sometimes called the preheader, sometimes the snippet — is the first bit of body copy that mail clients pluck out and display next to your subject. Gmail shows around fifty characters of it. Apple Mail shows more. Outlook shows less. But every major inbox client shows some, and every subscriber reads the two together, whether they realise it or not.

And most of the time, what they read is the subject line paired with something like: "Having trouble viewing this email? Click here." Or worse, "Unsubscribe · Update your preferences · View in browser." A perfectly good subject line, followed by a preview text that reads like the fine print on a receipt.

What the preview text is actually doing

Think of the two lines as a couplet. The subject asks a question or makes a promise. The preview either extends that promise, contradicts it, or ignores it. The good news is that when you write them together, deliberately, you get roughly twice the amount of copy in the inbox that other senders have. The bad news is that most senders are effectively writing half an email while their subscribers read a whole one.

The rule I use is small and I have never seen it fail. Read the subject and preview together, out loud, before you send. If the two of them together sound like a whole thought — a sentence, a promise, an argument — the campaign will open at a higher rate than if the two are disconnected. That is really the entire technique. Everything else is decoration.

A quick tour of common failures

• The default failure: no preview text was written, so the client falls back to the first line of the email, which is usually a "hi there" or the alt text of the header image.
• The tool failure: the ESP's template shoves boilerplate ("View in browser") at the top of every email, and that boilerplate becomes the preview.
• The clever failure: someone got creative and made the preview a punchline that only makes sense if you already opened the email.
• The redundant failure: the preview repeats the subject line verbatim, doubling the same words instead of adding new ones.

All four are common. All four leak opens. And all four are fixed by the same small habit — writing the preview text on purpose, at the same time you write the subject, treating them as one job.

What good looks like

A newsletter I read every Friday sends subject lines that are usually a single noun phrase — the topic of the week. The preview text always adds the argument. "The unsubscribe page." "And why it might be the most valuable page in your entire program." Read the two together and you know what the email is about, what it will argue, and whether you want to open it. Nothing is wasted.

The mechanics are laughably simple. Most modern ESPs let you set the preview text in a small field just below the subject line field. It takes ten seconds to fill. And yet in the majority of email programs I look at, that field is empty on every campaign.

The subject line and the preview text are a couplet, not a heading and a caption. Write them together, or lose half the real estate.

A word on length

The tempting thing, once you understand that the preview text has weight, is to fill every character. Do not. The best preview text is under eighty characters and reads like the natural continuation of the subject line, not like a second attempt at getting attention. Long preview text gets truncated in the inbox anyway, and long anything reads as trying too hard.

Short subject. Short preview. Read together as one line. Send.

The email programs that get this right are almost always the ones written by people who read a lot of newsletters themselves. This is not a coincidence. Reading email carefully teaches you which inches of the inbox are wasted, and which are doing quiet work. The preview text is the quietest of the working inches.