XML in a Nutshell, 3rd Edition

Element Declarations

Every element used in a valid document must be declared in the document’s DTD with an element declaration. Element declarations have this basic form:

<!ELEMENT name 
               content_specification>

The name of the element can be any legal XML name. The content specification indicates what children the element may or must have and in what order. Content specifications can be quite complex. They can say, for example, that an element must have three child elements of a given type, or two children of one type followed by another element of a second type, or any elements chosen from seven different types interspersed with text.