Part I: Introduction

A Gentle Introduction to SGML

2.4: Defining SGML Document Structures: The DTD

2.4.1: An Example DTD

A DTD is expressed in SGML as a set of declarative statements, using a simple syntax defined in the standard. For our simple model of a poem, the following declarations would be appropriate: 
    <!ELEMENT anthology      - -  (poem+)>
    <!ELEMENT poem           - -  (title?, stanza+)>
    <!ELEMENT title          - O  (#PCDATA) >
    <!ELEMENT stanza         - O  (line+)   >
    <!ELEMENT line           O O  (#PCDATA) >
These five lines are examples of formal SGML element declarations. A declaration, like an element, is delimited by angle brackets; the first character following the opening bracket must be an exclamation mark, followed immediately by one of a small set of SGML-defined keywords, specifying the kind of object being declared. The five declarations above are all of the same type: each begins with an ELEMENT keyword, indicating that it declares an element, in the technical sense defined above. Each consists of three parts: a name or group of names, two characters specifying minimization rules, and a content model. Each of these parts is discussed further below. Components of the declaration are separated by white space, that is one or more blanks, tabs or newlines.

The first part of each declaration above gives the generic identifier of the element which is being declared, for example poem, title, etc. It is possible to declare several elements in one statement, as discussed below.