The DocBook DTD defines the structure of a DocBook document. It contains rules about how the elements may be used; and what the elements ought to be describing. For example: it is the DocBook DTD which states all warnings are to warn the reader (this is the definition of the element); and may not contain plain text (this is the content model--and the bit which forces you to wrap your warning text in a para or perhaps a list).
It is important that you download the version(s) that match your document. You may want to configure your system now to deal with "all" DocBook DTDs if you are going to be editing older LDP documents. If you are a new author, you only need the first one listed: XML DTD for DocBook version 4.2.
The XML DTD is available from http://www.oasis-open.org/xml/4.2/. The LDP prefers this version of the DocBook DTD.
If you are going to be working with SGML versions of DocBook you will need one (or both) of: http://www.oasis-open.org/docbook/sgml/4.1/docbk41.zip or http://www.oasis-open.org/docbook/sgml/3.1/docbk31.zip
Example B-7. "Installing" DocBook Document Type Definitions
Create a base directory to store everything such as /opt/local/sgml/. Copy the DTDs into a sub-directory named dtd.
|Do not edit DTD files
The DocBook standard is described in these files. If you change these files, you are no longer working with DocBook.