"Re" appears in subject lines, memo headers, and formal letters constantly. Most people use it on instinct, assuming it means something like "reply" or "regarding." The actual answer is a little more interesting, and knowing it helps you use "re" correctly rather than reflexively.
What "re" actually means
"Re" is a Latin preposition meaning "in the matter of" or "concerning." It comes from the Latin word res, meaning "thing" or "affair." So "Re: your invoice" translates closely to "Concerning your invoice" or "In the matter of your invoice." That's it. No abbreviation, no shortcut for "reply." Just a preposition that English business writing borrowed from Latin centuries ago.
The confusion with "reply" is understandable. Email clients place "Re:" at the front of a reply subject line automatically, which trained a whole generation to read it as a reply marker. The email convention is a separate origin story, though: it almost certainly reinforced the Latin usage rather than replacing it.
Where "re" shows up in business writing
"Re" turns up in three main contexts. First, in formal letters and memos, it appears as a reference line between the salutation and the body: "Re: Contract Amendment No. 4." Second, in email subject lines, writers sometimes type it manually to signal the topic: "Re: Q3 budget review." Third, in legal and financial documents, "re" marks the subject of the document before the body text begins.
Each context follows the same logic. "Re" introduces the subject of what follows. Think of it as a label, not a sentence opener.
Should "re" be capitalised and punctuated?
In practice, three spellings coexist: "Re:", "RE:", and "re:". The colon is standard and almost universal. It signals that what follows is a label rather than a complete phrase. Drop the colon and "Re your invoice" reads oddly; keep it and the meaning is immediately clear.
As for capitalisation, "RE:" (all caps) is common in memo headers and legal documents, where it draws the eye quickly. "Re:" (sentence case) is more common in general business letters and emails. Both are correct. Capitalization in titles and headings follows style-guide conventions rather than fixed laws, and the same flexibility applies here: match whatever style your organisation uses, and stay consistent within a single document.
Is it an abbreviation?
No. "Re" is not an abbreviation for "regarding," "reference," or "reply." It's a complete Latin word used directly in English. You won't see it written "re." with a full stop (period) in the way that abbreviations like "approx." carry one. Some style guides allow for a period, but the mainstream convention omits it.
This matters because writers sometimes use "re" and "regarding" interchangeably in the same sentence: "Re: regarding your invoice." That's redundant. Pick one. In a subject line or reference line, "re:" does the job. In body text, "regarding" usually reads more naturally.
When to use "re" and when to skip it
Use "re:" in a subject line or reference line when you want a clean, scannable label for what follows. Legal and formal business correspondence uses it as standard. Memos almost always carry it.
Skip it when the subject line or topic is already self-evident. "Budget meeting Thursday" doesn't need "Re: Budget meeting Thursday." The extra word adds nothing. In email especially, clear subject lines are worth more than convention for its own sake. How you open a message already signals the tone; the subject line's job is clarity, not formality signalling.
Also skip "re" in running prose. Writing "Re the meeting last Tuesday" in the middle of a paragraph is informal to the point of seeming careless. Write "regarding the meeting" or "about the meeting" instead. "Re" belongs at the top of a document or in a clearly labelled field, not mid-sentence in body text.
Common mistakes with "re"
The biggest mistake is treating "re" as interchangeable with "reply." If you're starting a fresh email rather than responding to one, typing "Re: your question" is technically fine as a subject label, but it can confuse readers who assume you're answering something. Use a subject line that describes the actual content instead.
A second mistake: stacking "re" with "regarding" in the same line, as noted above. Choose one. A third mistake is omitting the colon: "Re your order" is missing the punctuation mark that separates the label from what it labels. The colon does functional work here, not decorative work.
A fourth issue comes up in formal email correspondence: some writers carry the "Re:" convention from email subject lines into the body text, opening a paragraph with "Re: the points you raised." In body text, that reads as jargon. Write "On the points you raised" or "Regarding your earlier comments" and the sentence immediately sounds more considered.
A note on legal usage
In legal documents, "re" or "in re" has a specific technical meaning. "In re" (Latin: "in the matter of") appears in the titles of court cases and legal proceedings, particularly in bankruptcy or probate matters where there's no opposing party: In re Estate of Smith. This is a distinct usage from business correspondence. If you're writing legal documents, follow the conventions of your jurisdiction and style guide rather than general business norms.
For everyday professional writing, the simpler summary holds: "re" means "concerning," it lives at the top of a document or subject line, it takes a colon, and it's a complete word rather than an abbreviation. Use it where convention calls for it, and write "regarding" when you need the same idea in running text.