The main.sgml file is the wrapper for each book. It defines the main entities of the book, and brings them together. (For more information on entities, see the Entities section.) Here is a typical main.sgml file:

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.1//EN" [

<!ENTITY % commonentities SYSTEM "../../i/common/entities.sgml"> %commonentities;
<!entity ccman            SYSTEM "../../i/connect/cc_man.sgml">
<!entity index            SYSTEM "indexCascadeConnect.sgml">

]>

<book id="bookcc">
&ccman;
&index;
&cccolophon;
</book>

The first line is the beginning of the SGML document type declaration. The only thing that ever changes on this line is the DocBook version number. That must be updated whenever you update the DocBook DTD. The next few lines are entity declarations, which are actually part of the document type declaration.

The first entity declaration is for a parameter entity that points to a common entity file that holds all the entities that are common to all of our books. The next two entities point to the SGML source files that hold SGML code for this book.

Finally, you see the book itself, whose content is included by calling one entity, and the index by calling another (explained below).