For some applications, DocBook “out of the box” may not be exactly what you need. Perhaps you need additional inline elements or perhaps you want to remove elements that you never want your authors to use. By design, DocBook makes this sort of customization easy.

It is even easier to customize DocBook 5.0 than it was to customize earlier releases. This is because DocBook 5.0 uses RELAX NG to express its schema. RELAX NG provides better support for modifications than DTDs, and the DocBook schema takes full advantage of that support.

This chapter describes the organization of the RELAX NG schema for DocBook and how to make your own customization layer. It contains methods and examples for adding, removing, and modifying elements and attributes, and conventions for naming and versioning DocBook customizations. It assumes some familiarity with RELAX NG. If you are unfamiliar with RELAX NG, you can find a tutorial introduction in the RELAX NG Tutorial [RNG-Intro].

You can use customization layers to extend DocBook or subset it. Creating a schema that is a strict subset of DocBook means that all of your instances are still completely valid DocBook instances, which may be important to your tools and stylesheets, and to other people with whom you share documents. An extension adds new structures, or changes the schema in a way that is not compatible with DocBook. Extensions can be very useful, but might have a great impact on your environment.

Customization layers can be as small as restricting an attribute value or as large as adding an entirely different hierarchy on top of the inline elements.