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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, the built-in mail tool of the web browser Opera, Microsoft Outlook and Outlook Express, 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.
Signatures in Usenet postings 
Signature blocks are also used in the Usenet discussion system. The Usenet standards specify that a signature block should be displayed as plain text in a fixed-width font (no HTML, images, or other rich text), and should 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", "sig separator" and "signature delimiter", 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. It is also recommended that a signature block should contain 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.
E-mail signatures in business 
Many corporations have internal policies requiring outgoing e-mails to have lengthy "signatures" appended to them, listing dozens of contact methods, disclaiming legal liabilities, notifying of virus scanning methods, and so forth.
Germany has laws requiring companies to disclose their company name, registration number, place of registration etc. in e-mail signatures. Ireland's Director of Corporate Enforcement requires all limited companies operating websites to disclose such information in their e-mails. The UK's ECommerce Regulations require this information in all emails from limited companies as well. While criticized by some as overly bureaucratic, these regulations only extend existing laws for (paper) business correspondence to email.
Internet forums 
On web 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 an elaborately-constructed HTML piece. Images are often allowed as well, including dynamically updated images usually hosted remotely and modified by a server-side script.
Signatures are seen as an art form by many of their creators, and there are many websites centered around their creation and display. Some of these websites have competitions, battles and signature of the week contests, where members submit their entries to have them featured on the website.
With FidoNet, echomail and netmail software would often add an origin line at the end of a message. This would indicate the FidoNet address and name of the originating system (not the user). The user posting the message would generally not have any control over the origin line. However, single-line taglines, added under user control, would often contain a humorous or witty saying. Multi-line user signature blocks were rare.
See also 
Notes and references 
- See Charles H. Lindsey: Usenet Best Practice, 2005, and also RFC 3676: The Text/Plain Format and DelSp Parameters, section 4.3, 2004, and RFC 1849: “Son of 1036”: News Article Format and Transmission, section 4.3.2, 2010 (first circulated in 1994).
- strictly speaking, only in emails concerning business dealings
- "German bureaucracy, coming to your e-mail now"
- "Irish ODCE Information Notice I/2007/2 - Disclosure of Company Particulars in E-Communications and on Websites"
- "ECommerce Regulations"
- "FidoNet EchoMail Specification"
- "A Proposed Replacement For FTS-0004"
- RFC 1855, "Netiquette Guidelines" (section 2.1.1 contains guidelines on mail)
- RFC 3676, (4.3 Usenet Signature Convention)