As discussed in the previous section, to be a well-formed XML document a set of rules must be adhered to. Many documents also conform to another set of rules that constrain what, how, and the frequency of elements and attributes. An XML document that is well-formed and conforms to a schema or DTD is called a Valid XML document.

This unit will teach the basic principles of one of these rule sets:  Document Type Definition (DTD). DTDs were originally developed to work with SGML and do not use SGML or XML syntax. Every SGML document instance was required to have a DTD.  XML is more flexible and does not require documents to conform to a DTD or schema (i.e. they are well formed but not valid). Schema languages include XML Schema, RNG Schema or Schematon. However, the underlying modelling principles are very similar and we teach DTDs here because it is a simpler syntax to learn. The TEI P4 guidelines had its own chapter on the use of DTDs [TEI P4 Documentation, Chapter. 3].

A DTD is a series of statements that can be written either at the beginning of an XML document (an internal DTD right after the XML declaration, or in a separate file from the XML document (an external DTD). The main advantage of an external DTD is that the rules defined in an external DTDs can be applied to many XML documents (called, in this case, document instances). This facilitates consistency of encoding and maintenance of the DTD. For the following examples and exercise we will use internal DTDs, because we are only working with one XML document. However, the same DTD statements would also apply to  external DTDs.

Further reading

W3schools, DTD Tutorial, http://www.w3schools.com/xml/xml_dtd_intro.asp