A signature block (often abbreviated as signature, sig block, sig file, .sig, dot sig, siggy, 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 and in a reply message of indicating that no more response follows. 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. Note that a sig block is not the same as a digital signature. A sig block is easily forged, whereas a digital signature uses cryptographic techniques to provide verifiable proof of authorship.
Information usually contained in a signature block includes the poster's name, phone number and email address, along with other contact details if required, such as URLs for sites owned or favoured by the author. A quotation is often included (occasionally automatically generated by such tools as fortune), or an ASCII art picture. Strict rules of capitalization are not followed. Among some groups of people it has been common to include self-classification codes, though the practice is waning.
E-mail and Usenet
An e-mail signature is a block of text appended to the end of an e-mail message often containing the sender's name, address, phone number, disclaimer or other contact information. Most e-mail clients, including Mozilla Thunderbird and Eudora, can be configured to automatically append an e-mail 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 e-mail servers can be configured to append e-mail signatures to all outgoing mail as well. However, when multiple replies to the same post occur, care should be taken to prevent multiple signatures from building up so that message lengths remains legible and message size manageable.
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 McQuary 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 can display the sig block properly.
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", "sig-marker", and "sig separator", allows software to automatically mark or remove the sig block as the receiver desires. The signature prefix chosen can be different for different people serving as a distinguishing feature of their signatures. A correct delimiter is required for a news posting program to receive the Good Netkeeping Seal of Approval.
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