The structure of SGML documents

1.

The document instance:

• contains data (text)
• contains markup
• must contain/refer to a DTD

2.

DTD, the document type definition:

• defines the structure of a document
• may be stored externally to a document

3.

The SGML declaration:

• usually common to all documents in an SGML installation
• if it isn't a part of the document, the default Reference Concrete Syntax (RCS) is used