SGML and MARC

"SGML"

stands for Standard Generalized Markup Language. The standard was issued by the International Organization for Standardization (ISO) in 1986. It allows publishers and authors to represent documents in such a way that the text may be separated from the structure without regard to the particular word-, text- or media-processing system being used. Documents which conform to the SGML format may be exchanged and processed on many different systems in many different ways. This represents an enormous advance over traditional word processors (e.g., Microsoft Word, WordPerfect, Displaywrite, etc.), which mix commands that affect visual properties with the text or content of the document, resulting in a "closed" environment that thwarts one's ability to interchange and integrate documents when they were created on various systems. Let's continue refining our definitions....

SGML

is a language for specification of markup languages (like HTML, TEI, etc.)
is a set of rules--syntax and semantics--for designing markup languages
is a system of "metalanguage" constructs
is a kit for constructing text-description (markup) languages
is flexible enough to define an infinite number of markup languages (one for memos, one for books, one for hypermedia, etc.)

DTD

means Document Type Definition
is an SGML application (i.e., is written "in SGML")
defines the structure of a particular type of document
more precisely, a DTD defines a specific set of tags and markup commands (element names, whether they are repeatable, what order they should be in, what kinds of markup can be omitted, the contents of elements, tag attributes and their default values, names of permissible entities, and typewriter conventions that may be used).
may be stored externally to the document

A Document instance

is the document itself which:

contains data (contents)
contains markup
includes a reference to the DTD (if it is external to the document)

HTML

HyperText Markup Language
is an SGML-conforming markup language
is the initialism often displayed as the DTD declaration in the markup at the top of a hypermedia document
allows different Web browsers to interpret the appearance of a single document differently (because, since HTM language conforms to SGML, the structure commands are indicated separately from the text, data, graphics, etc.).

United States Patent and Trademark Office Home Page
(URL=http://www.uspto.gov/)

is an example of a document instance
is written "in HTML"
when displayed in a Web browser's "source" view, shows as the DTD declaration at the top of the document.

MARC

stands for MAchine Readable Cataloging
is a language that does not conform to SGML
is a language which allows library [archives, museum, etc.] information to be stored, shared, and manipulated by computer in a consistent manner, regardless of the variations in resulting appearances of the data in different online systems.
has various applications, or formats (e.g., formats for bibliographic data, for authorities data, for holdings and locations, for classification data--there's even a format for community information!)
each format defines a specific set of fixed-length and variable-length fields, and tags, indicators, and subfield codes useful for processing the text/content of different elements (e.g., title) and attributes or properties (e.g., language, physical form, target audience) of the entities described in bibliographic and related records.
USMARC is a widely-accepted standard which itself is based on ANSI Z39.2, American National Standard for Bibliographic Information Interchange, first promulgated in 1971, and revised in 1985. (Note the words "information interchange" in the standard's title, and consider how this parallels the objectives of SGML.)
was developed originally to support library automation, whereas SGML was developed to enhance automation in the publishing/authorship industries.

Ms Stone wishes to acknowledge the following people or sources for contributing to this introduction: Thomas Bruce (Cornell Law School) and Chris Corcos and Judy Kaul (both at Case Western Reserve University Law Library), who shared some handouts on HTML and Web publishing; Eric van Herwijnen, who authored Practical SGML 2nd ed. (Kluwer Academic, 1994); Walt Crawford, the author of MARC for Library Use 2nd ed. (G.K. Hall, 1989); and the following readers of the newsgroup "comp.text.sgml" who offered guidance on why it is incorrect to call HTML a subset of SGML: Ray Dassen, Dept. of Mathematics & Computer Science at Leiden University, The Netherlands; C.M. Sperberg- McQueen, ACH/ACL/ALLC Text Coding Initiative at University of Illinois at Chicago; and David Durand, Boston University Computer Science.
Any errors or misstatements in the text above are entirely the responsibility of Ms Stone, however, and do not necessarily reflect the opinions of other scientists or authors.

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Send comments or suggestions to: Alva T. Stone