Use RE: As a Reply in Emails

RE: has different meanings in paper and electronic communications

Close-Up Of Fountain Pen On Paper

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Back when all messages were delivered on paper, the term Re: stood for "in regards to," or "in reference to." It is not an abbreviation; in fact, it is taken from the Latin In re which means "in the matter of." In res is still used in legal proceedings that is uncontested and lacks formal adverse parties.

With the advent of electronic communications, however, the use of RE: has taken on a repurposed meaning in a way that helps to keep email conversations clear and organized for recipients. RE: in an email is used in the subject line, preceding the subject itself, and it indicates that this message is a reply to the previous message under the same subject line.

This helps users recognize messages and responses that are on a particular topic, which is especially helpful if a person is engaged in multiple different email conversations at the same time. 

When RE: Causes Confusion in Emails

If you put RE: in front of the subject line of a new message that is not a reply to an older message, the recipients may be confused. They may think the reply belongs to an email thread to which they have not been privy or perhaps do not belong, or that the preceding messages in the conversation were not received for some reason.

Regardless of what might be true in other contexts, in email correspondence the Re: is no longer meant as "regarding the subject of" — the email line already contains the label Subject: to indicate the topic of the message.

Use RE: For Reply

To prevent confusion, avoid using RE: in the subject line unless the message is a reply to a message with that specific subject line. RE: should only be used when making replies in email.