The subject line in a business letter does one job: it tells the reader what the letter is about before they read a word of the body. Get it right and your letter is faster to route, easier to act on, and harder to misfile. Get it wrong and your reader starts with a question instead of an answer.
What a business letter subject line actually is
A subject line (sometimes called a re: line) appears between the salutation and the opening paragraph. It is not a heading for the entire letter. It's a single-line summary that names the specific matter the letter addresses. Think of it as a label, not a title.
Many writers confuse the subject line with a business letter reference line, which carries a file number, contract identifier, or case code. They're related but distinct. The reference line identifies a record. The subject line describes the topic in plain language. A letter can carry both, placed in that order: reference line first, subject line directly beneath it.
Where to place it in the letter
Standard placement puts the subject line two lines below the salutation. Like this:
- Recipient's address
- Date line
- Salutation (Dear Ms Hartley,)
- Subject line
- Opening paragraph
Some older style guides place the subject line above the salutation, between the inside address and the greeting. Both conventions exist. If your organisation follows a specific business letter format, follow that. If you're starting from scratch, place it below the salutation: that's the more widely used position in current practice.
How to format it correctly
There are three accepted formats, and all three work. The choice depends on your house style.
Bold, no label: Simply write the subject in bold text. No prefix. This is the cleanest option and the most common in modern letters.
"Re:" prefix: Write "Re: [subject]" in plain or bold text. The word re is a Latin preposition meaning "in the matter of," not short for "regarding" or "reply." Using it this way is correct and still widely accepted in legal, financial, and government correspondence.
"Subject:" prefix: Write "Subject: [subject]" in plain or bold text. This is most common in formal institutional letters where clarity of structure matters above style.
Pick one format and stick to it throughout a document series or correspondence thread. Switching formats between letters in the same exchange looks careless.
What to write in the subject line
The subject line should be one line. Not a sentence. Not a heading with a colon and three sub-clauses. One concise phrase that names the matter precisely.
Good examples:
- Renewal of Service Agreement – Contract 4471
- Notice of Redundancy: Michael Chen
- Complaint: Invoice 2908, 14 August 2026
- Request for Planning Permission, 22 Orchard Lane
Bad examples:
- "Important information regarding your account and recent changes to our billing schedule" (too long)
- "Further to our discussion" (says nothing specific)
- "Please read this letter" (not a subject, it's a command)
If you find yourself writing more than 10 words, you're describing the letter, not naming the subject. Cut.
Capitalisation rules
Most style guides recommend title case for business letter subject lines: capitalise the first word and all major words. Some organisations use sentence case (first word only). The key is consistency. Whatever you choose, don't mix the two within the same letter or batch of correspondence.
Do not write the subject line in all capitals. ALL CAPS reads as shouting, and it's harder to scan quickly, which defeats the purpose of having a subject line at all.
Does every business letter need a subject line?
No. Routine, short letters with a clear context don't always need one. A brief letter of thanks following a meeting, or a simple letter of introduction, can open directly with the salutation and body without any subject line and read perfectly well.
Subject lines earn their place in letters that deal with a specific matter requiring action, reference, or filing. Contracts, complaints, formal notices, applications, legal correspondence, and financial letters all benefit from one. When in doubt, include it: it costs nothing and helps the reader immediately.
A note on letters with both a subject line and a postscript
Some writers ask whether a subject line makes a business letter postscript redundant. The answer is no. The subject line sits at the top and frames the entire letter. A postscript sits at the bottom and draws attention to a single point the writer wants to emphasise after the main body. The two serve different purposes at different points in the reader's experience.
A well-structured letter can carry both: the subject line sets context, and the postscript closes on a specific call to action or an important note that might otherwise get lost. Neither replaces the other.
Quick checklist before you send
Before the letter leaves your desk, check the subject line against these four questions:
- Does it name the specific matter, not just the general topic?
- Is it one line, no longer?
- Does the format (bold, Re:, Subject:) match your house style?
- Would a stranger reading only the subject line know what the letter is about?
If the answer to every question is yes, the subject line is doing its job. If any answer is no, revise it before the letter goes out.