Document Type Definition is one of several SGML and XML schema languages, and is also the term used to describe a document or portion thereof that is authored in the DTD language. A DTD is a formal expression (in XML) of the structural constraints for a class of XML documents. The DTD language constructs are element and attribute-list declarations. The first versions of DDI Codebook (those released before DDI Codebook 2.5) used a DTD rather than XML schema.

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