A Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes.

A DTD can be internally defined e.g.

<!DOCTYPE root-element [
  element-declarations
]>

or externally defined e.g.

<!DOCTYPE root-element SYSTEM "filename.dtd">

where the element-declarations are in filename.dtd

A simple example of DTD element-declarations

  <!ELEMENT note (to,from)>
  <!ELEMENT to      (#PCDATA)>
  <!ELEMENT from    (#PCDATA)>

This says that note contains 2 child elements and to and from are of type #PCDATA .

An element is defined as

<!ELEMENT element-name category>

or

<!ELEMENT element-name (element-content)>

category can be either EMPTY or ANY

element-content can be any combination of #PCDATA or child elements

child_element+ - one or more occurrences

child_element* - zero or more occurrences

child_element? - zero or one occurrences

(a|b) - Either a or b (can be #PCDATA)

a,b - a and then b in that order

An attribute specified in a DTD takes the form

<!ATTLIST element-name attribute-name attribute-type default-value>

for example:

<!ATTLIST payment
   currency CDATA "US Dollars"
   amount CDATA #REQUIRED>
   form (Cash|Cheque) "Cheque" 

Entities are variables used to define shortcuts to standard text or special characters.

Entities are declared in DTDs and can be declared internal e.g.

<!ENTITY writer "Donald Duck."> 

or external e.g.

<!ENTITY writer SYSTEM "http://a.com/entities.dtd">

Entity references are references to entities and may be used in XML documents.

An entity reference has three parts: an ampersand (&), an entity name, and a semicolon (;). e.g.

<author>&writer;</author>