What is the role of preview text as “second subject line”?

Preview text is the short snippet that appears after your subject line in the inbox. On mobile it's often on the same line. On desktop it shows just below or to the right, in lighter text. Most readers see it before they open. That makes it a second subject line slot that most senders treat as an afterthought.

The most common mistake: letting it default. If you don't set preview text deliberately, your email client will pull the first readable text from your email body. That might be "View this email in your browser" or an accessibility description or a navigation link. Nothing you'd choose to show a subscriber who's deciding whether to open.

What to put there instead: let it extend your subject line, not repeat it. If your subject sets up a question, your preview text can tease the answer. If your subject announces a sale, the preview can add the specific discount or deadline. Together, subject and preview give you roughly 100-150 characters depending on the client. Gmail tends to show more. Outlook mobile shows less. Write for the shorter end and you'll be safe across most clients.

A few things worth knowing: if your preview text is very short, the email body content can bleed through beneath it in the inbox. You can pad it with hidden whitespace to prevent that. And yes, you can use preview text to be a little playful or subvert expectations. Subject: "You messed up." Preview: "Just kidding. Here's what's actually going on." That kind of contrast can work for the right audience.

Treat the subject and preview as a pair. Write your subject first, then write the preview to complete the pitch. Test combinations: different subjects work better with different preview approaches. Once you find a pairing pattern that works for your audience, you'll have a repeatable edge on every send.