Document Type Definition
Each DTD element must either be a container element, or be empty (a place holder). Container elements may contain text, child elements, or a mixture of both.
DTD also specifies the names of attributes, and dictates which elements they may appear in. For each attribute it specifies whether it is optional or required.
DTD tree describing a book as containing a number of Chapter elements, with each chapter containing either a number of Paragraph elements, or a single Sections element
A particular document tree has a node for each actual chapter and paragraph present, and may omit some of the optional elements