The reference line is one of the smallest parts of a business letter and one of the most mishandled. Writers either skip it entirely, format it inconsistently, or confuse it with the subject line. None of those outcomes serves the reader. A correctly placed reference line tells the recipient exactly how to route, file, or respond to your letter before they read a single sentence of the body.
What a reference line actually does
A reference line identifies an existing account, case, file, contract, or correspondence that the letter relates to. It does not summarise the letter's purpose (that's the subject line's job). It points to a record. A letter about a disputed invoice might carry a reference line that reads Re: Invoice #INV-2024-0447. A legal letter might reference a case number. An HR letter might cite a contract ID.
Think of it as metadata made visible. In large organisations, letters pass through several hands before reaching the right desk. The reference line is what routes them correctly, without anyone having to read the body first.
Where to place it
Placement depends on whether your letter uses a subject line as well. If the reference line is the only identifying element, it sits two lines below the recipient's address block and two lines above the salutation. If you're using both a reference line and a subject line, the reference line comes first, followed by one blank line, then the subject line, then one blank line, then the salutation.
Left-aligned is the modern standard. Older block formats occasionally centred both lines, but left-alignment is now correct for virtually every professional context. If you're working from a template, check the proper business letter format guidelines to make sure your document structure is consistent before you add the reference line.
How to write it
Keep the reference line short. Its purpose is identification, not explanation. Three formats are widely accepted:
- Re: [identifier] — the most common form in British and Australian business writing
- Ref: [identifier] — common in formal and legal correspondence
- Reference: [identifier] — spelled out in full when the document is very formal or when the abbreviation might confuse international readers
Follow the label with the actual reference: an invoice number, file number, policy number, project code, or previous letter date. Never use vague references like "Re: our recent conversation." That tells the reader nothing and defeats the purpose entirely.
Reference line vs subject line: the difference
Writers frequently conflate the two, partly because both can appear in the same position and both often start with "Re:". The distinction matters. A subject line announces what the letter is about, in plain language: "Subject: Changes to your payment schedule." A reference line points to a record: "Re: Account #004-2290-B."
You can use both in the same letter. Government departments and legal firms do this regularly. When you do, the reference line anchors the letter to a file; the subject line orients the reader to the content. Keep them on separate lines and don't merge the two into one run-on identifier. If you're uncertain about how the date line and other header elements should be sequenced, work from the top of the letter down: date, then recipient block, then reference and subject, then salutation.
Capitalisation and punctuation
Capitalise only the label and any proper nouns or identifiers in the reference itself. Don't capitalise every word as if it were a title. "Re: Policy renewal — Group life cover No. 7821" is correct. "Re: Policy Renewal – Group Life Cover No. 7821" over-capitalises and looks bureaucratic rather than professional.
A colon after the label is standard. No full stop at the end of the line. If the reference contains a number, write the number exactly as it appears in the original document: maintaining the format the recipient uses in their own system means their staff can look it up without reformatting it.
When to include one (and when not to)
Use a reference line whenever the letter relates to an existing file, account, case, or prior correspondence. This covers most formal business letters, nearly all legal correspondence, insurance and financial documents, and any complaint or dispute letter. Include it if you're replying to a letter that carried one.
Skip the reference line when the letter is introductory or exploratory, and no existing record exists. A prospecting letter to a potential client doesn't have a file to reference. A cold introduction letter has no case number to cite. Adding a blank or meaningless reference line in these situations looks like filler. The closing paragraph does more to move introductory letters forward than any invented reference ever could.
A quick checklist
Before sending any formal business letter, run through these four questions for the reference line specifically:
- Does the reference point to a real, existing record that the recipient can look up?
- Is it placed correctly: after the recipient block, before the salutation?
- Does it use a consistent label ("Re:", "Ref:", or "Reference:") across all outgoing letters?
- Is the identifier copied exactly from the original document, including case and formatting?
Getting each of these right takes under a minute. Missing any of them can slow down the very routing and filing process the reference line was meant to accelerate.