The Document Type Definition

Each SGML document must refer to a DTD.

The Document Type Definition consists of:

• the formal part

  formally specifies what markup can appear in the document type and in what relation to other markup and data

• the documentation

  specifies what the defined elements mean:

  <date>Joe</date> can be formally OK, but is nonsense.

The formal DTD consists of:

• ELEMENT declarations
• ATTRIBUTE declarations
• ENTITY specifications