A signature block (often abbreviated as signature, sig block, sig file, .sig, dot sig, siggy, or just sig) is a personalized block of text automatically appended at the bottom of an email message, Usenet article, or forum post.

An email signature is a block of text appended to the end of an email message often containing the sender's name, address, phone number, disclaimer or other contact information.

"Traditional" internet cultural .sig practices assume the use of monospaced ASCII text because they pre-date MIME and the use of HTML in email. In this tradition, it is common practice for a signature block to consist of one or more lines containing some brief information on the author of the message such as phone number and email address, URLs for sites owned or favoured by the author—but also often a quotation (occasionally automatically generated by such tools as fortune), or an ASCII art picture. Among some groups of people it has been common to include self-classification codes.

Example of a signature block using ASCII art.

Most email clients, including Mozilla Thunderbird, Opera Mail, Microsoft Outlook, Outlook Express, and Eudora, can be configured to automatically append an email signature with each new message. A shortened form of a signature block (sometimes called a "signature line"), only including one's name, often with some distinguishing prefix, can be used to simply indicate the end of a post or response. Most email servers can be configured to append email signatures to all outgoing mail as well.

An email signature generator is an app or an online web app that allows users to create a designed email signature using a pre-made template (with no need for HTML coding skills).

Signature blocks are also used in the Usenet discussion system.

— a real example from Theodore Ts'o during the Tanenbaum–Torvalds debate