A signature block (often abbreviated as signature, sig block, or just sig) is a block of text automatically appended at the bottom of an e-mail message, Usenet article, or forum post. This has the effect of "signing off" the message. A common practice is to have one or more messages containing some brief information on the author of the message.

Information usually contained in a sig block includes the poster's name and email address, along with other contact details if required, such as URLs for sites owned or favoured by the author. A witty or profound quotation is often included (occasionally automatically generated by such tools as fortune), or an ASCII art picture.

E-mail and Usenet

Since by definition these blocks are added automatically to a message, usually regardless of its content, there are guidelines of netiquette regarding their size. The most common guideline, called the McQuarry limit, is a size of no more than four lines of less than eighty columns each. This keeps the overall size of the message down, conserving bandwidth as well as the time required to read the message, and ensures that eighty-column terminals (the most common terminal width by far) can display the sig block properly, allowing for programs that reserve the last column for a continuation character; using all eighty columns for text can result in a character wrapping to the next line.

The formatting of the sig block is prescribed somewhat more firmly: it should be displayed as plain text in a fixed-width font (no HTML, images, or other rich text), and must be delimited from the body of the message by a single line consisting of exactly two hyphens, followed by a space, followed by the end of line (i.e., "-- \n"). This latter prescription, which goes by many names, including "sig dashes", "signature cut line", and "sig-marker", allows software to automatically mark or remove the sig block as the receiver desires. A correct delimiter is required for a news posting program to receive the Good Netkeeping Seal of Approval.

However, whether due to ignorance or disregard for these guidelines, a great many people use sig blocks that are either formatted improperly or larger than these suggested dimensions. In past decades, such practice was referred to as warlording, named for one particular Usenet regular who openly flouted the guidelines with sig blocks stretching to many hundreds of lines of ASCII art on messages with little or no relevant content.

Many corporations have internal policies requiring outgoing emails to have lengthy "signatures" appended to them, listing dozens of contact methods, disclaiming legal liabilities, notifying of virus scanning methods, and so forth. These corporate signatures are almost universally large (often larger than the message itself), and composed without regard for the netiquette guidelines described above; they are seen as obnoxious and irritating by many who receive them.

Forum

On forums, the rules are often less strict on how a signature block is formatted, as Web browsers typically are not operated within the same constraints as text interface applications. Users will typically use a "signature" text area in their given profile for input, which can then allow a user to turn off signatures. Depending on the board's capabilities, signatures may range from a simple line or two of text to elaborately-constructed HTML and CSS monstrosities that can be in excess of four kilobytes. Images are often allowed as well. Teenagers, in particular, will sometimes push the limits of forum signatures by including multiple screenfuls of large images pertaining to their pop-cultural crushes and obsessions, to append to their messages which are often one-liners that say "Me Too!" to the previous poster.

A common ground rule is to keep a signature smaller than a typical post.