A DTD defines the structure of an XML document through element declarations that specify element types and attributes. It allows for uniformity across XML documents through constraints like requiring certain child elements. A DTD can be internal, embedded within the XML code, or external, stored separately in its own file. It uses markup declarations like <!ELEMENT> to define tags and their hierarchical tree structures.

1. DTD A Document Type Definition =A set of structural rules, declarations Use of DTD : impose uniform style sheet in external style sheet for XHTML Application program for testing document against the DTD

4. Declaring Elements <!ELEMENT element_name (list of names of child elements)> Example) <!ELEMENT memo (from, to, data, re, body)>

5. Tree structure memo from to data re body Leaf node, internal node : character pattern or a list of its child elements

6. Child element specification modifiers <!ELEMENT person (parent+, age, spouse?, sibling*)

7. Parsable character data a string of any printable character except less than(<), greater than(>), and ampersand(&). <!ELEMENT element_name(#PCDATA)> <!ELEMENT b (#PCDATA)> <!ELEMENT p (#PCDATA|a|b)*> EMPTY and ANY