Required - http://www.oasis-open.org/docbook/sgml/4.1/docbk41.zip or http://www.oasis-open.org/docbook/sgml/3.1/docbk31.zip
The XML DTD is available from http://www.oasis-open.org/xml/4.1.2/.
The DocBook DTD defines the tags and structure of a DocBook document. Modifying the DTD, such as adding a new tag, means that this DTD is no longer DocBook.