3. XML document model : DTD

XML provides a means of checking a document's syntax using DTDs (Document Type Definition). These files describe the structure of documents referring to them, using an appropriate language. All XML documents must scrupulously follow XML notation conventions, and may also refer to a DTD describing the nesting of possible elements. A document that respects the rules defined by the XML standard is called a well-formed document, and a well-formed XML document whose syntax conforms to the rules stipulated in a DTD is called a valid document.