XML 1.0 Document Type Definitions (DTDs) provide a particular vocabulary for constructing schemas. Schemas are tools for describing document structures and content. The World Wide Web Consortium (W3C) is currently developing an XML Schema Definition Language (XSDL) proposal that uses XML document syntax to describe XML document structures. XML Instance uses the term schema to describe both DTDs and the various other schema proposals, including DCD, DDML, XDR, SOX, and the schema standards currently under development at the W3C. XML Instance provides a consistent interface you can use when working with any of these schema formats, also known as XML schema dialects.
More information on the newer schema formats is available in their specifications. The list below includes links to documentation for each format.
In common usage, schemas are still contrasted to DTDs. Typically, schemas are portrayed as the 'next generation' beyond DTDs, although DTDs are really the 'first generation' of schemas. DTDs have much software support in XML 1.0-compliant parsers and applications. XDR and SOX are stable XML schema dialects. Parser support for XDR is available from http://msdn.microsoft.com/downloads/webtechnology/XML/msxml.asp. SOX is supported by Commerce One's XML Development Kit. XSDL is an emerging standard under development at the W3C.
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