Document type definitions (DTDs) define legal building blocks for XML documents. It uses a series of legal elements to define the structure of the document.

A DTD can be declared in an XML document as a line or as an external reference.

DTD - element syntax

In a DTD, elements are declared through element declarations.

DTD - element example

<!ELEMENT?element-name?EMPTY>
實例:
<!ELEMENT?br?EMPTY>
XML?example:
<br?/>