A one to n line ascii file which is appended to the end of email messages by mail programs such as Pine. Typically contains such information on the sender as name, email, homepage, often a ham radio callsign, a MBTI and sometimes a full postal address or other such location identifier. Suits may choose to include the most impressive version of their job title and corporate identity here. A short irreverent tagline, fortune, or other cookie is often included.

Unix users have historically tended to store this information in a file called .signature, which lead to it being referred to as either a "dot signature" or "sig file." Seasoned Internet users will find, with a few notable exceptions, that the experience and credibility of a correspondent tends to be inversely proportional to the number of lines in their signature.