This document provides an overview of SGML and XML. It describes SGML as a metalanguage that uses start and end tags along with elements that can contain other subelements in a hierarchical structure. XML is described as a type of SGML that is easier to process due to requiring end tags and other simplified features. Examples are given of how MARC21 records can be encoded in both SGML and XML formats. The development of XML by the W3C and alternatives to DTDs like XML Schema are also summarized.

1. SGML and XMLSGML and XML
Richard Sapon-White
Introduction to Metadata
May 13, 2013

2. SGMLSGML
Standardized General Markup Language, a
metalanguage
DTD (Document Type Definition) –
identifies the scheme used to define tags
and the rules for using them
DTDs: EAD, HTML, DTD for MARC21
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3. SGML (cont.)SGML (cont.)
Uses start and end tags; attribute names
and values
Elements may include text, text +
attribute, attribute
Elements (wrappers) may include other
elements (subelements)
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4. SGML Advantages and DisadvantagesSGML Advantages and Disadvantages
Advantages
◦ Element names can be mnemonic, e.g. EAD
element <titleproper> (compare with 245 $a)
◦ Hierarchical: can inherit data encoded at
higher levels
Disadvantage
◦ Difficult for programs to process; software
applications are complex
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5. XMLXML
Type of SGML with fewer features and
options, for example:
◦ End tags must be used
◦ Attribute values always in quotation marks
Easier to process
◦ Tags are case-sensitive
◦ Attributes may contain UNICODE characters
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6. Examples: LC DTD for MARC21Examples: LC DTD for MARC21
<mrcb245 i1=0 i2=4><mrcb245-a>The
health of dairy cattle /<mrcb245-c>edited
by Anthony H. Andrews. [SGML]
<mrcb245 i1=“0” i2=“4”><mrcb245-
a>The health of dairy cattle/</mrcb245-
a><mrcb245-c>edited by Anthony H.
Andrews.</mrcb245-c></mrcb245>
[XML]
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7. XML DevelopmentXML Development
World Wide Web Consortium (W3C)
actively developing XML
XML linking language, query language,
stylesheet language, etc.
XML Schema are an alternative to DTDs
LC has developed MARCXML, an XML
Schema for MARC21 records
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8. XML: TodayXML: Today
The language of the Web
Web browsers and other applications
support XML-encoded documents
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