Standard Generalized Markup Language

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The Standard Generalized Markup Language (SGML) is a metalanguage in which one can define markup languages for documents. SGML is a descendant of IBM's Generalized Markup Language (GML), developed in the 1960s by Charles Goldfarb, Edward Mosher and Raymond Lorie (whose surname initials were used by Goldfarb to make up the term GML^[1]).

SGML provides an abstract syntax that can be written in many different concrete syntaxes. If one defines a concrete syntax in the SGML Declaration, one does not need to use "angle brackets" as tag delimiters, although they are the norm — part of the reference concrete syntax defined in the standard. GML used a colon to introduce a tag, a period to end it, and 'e' to indicate an end tag: :xmp.thus:exmp., and SGML is flexible enough to accept that grammar, too.

Original uses

SGML was originally designed to enable the sharing of machine-readable documents in large projects in government, legal and industry, which have to remain readable for several decades—a very long time in information technology. It has also been used extensively in the printing and publishing industries, but its complexity has prevented its widespread application for small-scale general-purpose use.

A fragment of the OED (1985), showing SGML markup

Primarily intended for text and database publishing, one of its first major applications was the second edition of the Oxford English Dictionary (OED), which was and is wholly marked up using an SGML-like markup.

Syntax

SGML allows most aspects of a markup language's syntax to be customized.

The default syntax appears similar to this example:

<QUOTE TYPE="example"> 
  typically something like <ITALICS>this</ITALICS> 
</QUOTE>

HTML uses this SGML default syntax.

Customization of the syntax for a markup language in SGML is specified by a Document Type Definition, or DTD.

According to the reference syntax, letter case is not distinguished in tag names so the three tags <quote>, <QUOTE>, and <quOtE> are equivalent (a concrete syntax may change this rule through the NAMECASE NAMING declarations).

Whether a tag must be paired like the above <QUOTE></QUOTE> pair or occurring singly like an HTML <HR> is defined in the DTD for the markup language being defined (as long as the OMITTAG feature is enabled). In this case the XML counterpart would be the specific empty tag <hr/>, which is equivalent to the SGML NET-enabling start-tag, introduced in the TC2 (International Standard ISO 8879:1986, Technical Corrigendum 2, Nov. 1999).

SGML markup languages whose concrete syntax enables the SHORTTAG VALUE feature, do not require attribute values containing only alphanumeric characters to be surrounded by quote marks " (LIT) or ' (LITA), so that the above markup could be written:

<QUOTE TYPE=example> 
  typically something like <ITALICS>this</ITALICS> 
</QUOTE>

One feature of SGML markup languages is the NET (Null End Tag) construction: <ITALICS/this/ which is structurally equivalent to <ITALICS>this</ITALICS>. Another is the "presumptuous empty tagging", such that the empty tag </> in <ITALICS>this</> "inherits" its value from the nearest previous nonempty tag, which of course is <ITALICS> (in other words, it closes the most recently opened item). The expression is thus another, more concise, equivalent to <ITALICS>this</ITALICS>. A third is the 'text on the same line' feature, which allows an item to be ended by a line-end (especially useful for headings and the like).

Additionally, the SHORTTAG NETENABL IMMEDNET feature allows to shorten tags that surround an empty text value:

<QUOTE></QUOTE>

can be written as

<QUOTE//

Where the first "/" stands for the NET-enabling start-tag close (NETSC) and the second one stands for the NET. (Note: XML defines NETSC as "/" and NET and ">" hence, in XML, this construct looks as <Quote/>).

SGML is an ISO standard: "ISO 8879:1986 Information processing—Text and office systems—Standard Generalized Markup Language (SGML)" which was accepted in October of 1986.

Derivatives

HTML

Main article: HTML

HTML was originally designed based on SGML tagging but without SGML's emphasis on rigorous markup. It was later reformulated (at version 2.0) to be an application of SGML, although there's some debate on whether it ever actually became one. The charter for the recently revived World Wide Web Consortium HTML Working Group goes so far as to say, "the Group will not assume that an SGML parser is used for 'classic HTML'".^[2]

XML

Main article: XML

XML is an application of SGML, designed so as to make the parser much easier to implement, compared to a full SGML parser. A consequence of the ease of implementation is that XML, rather than SGML, is nowadays widely used for deriving document specifications. Contributing to this is also the fact that few SGML-aware programs existed when XML was created. The number of XML applications today is large. XML also has a lightweight internationalization. XML is used for general-purpose applications, such as the Semantic Web, XHTML, SVG, RSS, Atom, XML-RPC and SOAP.

DocBook

Main article: DocBook

Another markup language originally created as an application of SGML is DocBook, designed for authoring technical documentation. DocBook is now also available as an XML application.

Other

There are also a number of languages that are related in part to SGML and XML, but, because they cannot be parsed or validated or otherwise processed using standard SGML and XML tools, cannot be considered to be applications of SGML or XML. One example is the Z Format, a language designed for typesetting and documentation.

See also

• DSSSL - a Scheme-based processing language similar to XSL
• LaTeX
• List of general purpose markup languages
• Markup language
• Structured data interchange
• SGML entity

References

External links

• Overview of SGML Resources at W3C's website.
• Introduction and Examples of Software Documentation in SGML
• A gentle introduction to SGML
• SGML Syntax Summary by Charles Goldfarbar:SGML

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