Subject line

A subject line looks like copywriting and behaves like layout. It is never read on its own: it arrives in a list, set in type the sender did not choose, at a width the sender cannot measure, with the sender name above it and the preview text below it. The three are read as one sentence in the second or so a person spends deciding whether this row is worth a tap, which is why writing any one of them in isolation is the mistake. The subject is the middle clause.

The design constraint that follows is truncation. Every client and every window gives the line a different allowance, so the cut lands somewhere the sender will never know, and the only safe assumption is that it lands early. What that turns into is a rule about order rather than about length: the meaning has to have arrived before the cut. “Shipped: order 4471, arrives Friday” survives being sliced in half because the fact is at the front. “A quick update about your recent order with us” does not survive anything, because the words that carry information were saved for the end, and on a phone the end is not shown.

The rest is restraint. A subject that repeats the sender name wastes the only line that could have said something new, a subject that repeats the preview text wastes two. Length is worth less attention than sequence, since a long line that front-loads reads fine truncated while a short vague one reads as nothing at any width. And the line is a promise the mail has to keep: the words most likely to earn an open are also the words most likely to be resented if the body does not deliver them, which is the quiet reason curiosity gaps and false urgency cost more than they return.

Two habits show up in every list of tips worth reading. Write the line before the mail, because a subject you cannot write in eight words usually means the mail is about more than one thing. And read it in a narrow column with the sender and the preview beside it, at the size a phone will actually use, rather than in the wide field of a sending tool where every line looks complete.