XML Catalogs and DocBook SGML instances require either a public or system identifier to specify the schema. Public identifiers are abstract identifiers that must be mapped to a schema file to be useful. For the SGML versions of DocBook, the distribution included a catalog file, doc- book.cat that provided mappings for all of the public identifiers referenced by DocBook. Unlike SGML, which allows an instance to use only a public identifier, XML requires a system identifier in the form of a URI. For the XML version of DocBook, the distri- bution contains a sample catalog file using the XML Catalogs: OASIS Standard V1.1 [XML-CAT], which is used by many XML tools. To quote the standard, an XML catalog "defines an entity catalog that maps both ex- ternal identifiers and arbitrary URI references to URI references." This means that you can define mappings through an XML catalog that tell tools where you have put the schemas (and other files, like the stylesheets) on your system. Those tools can then access them without having to resolve a remote URL. Installing and setting up the DocBook XML catalog One way to set up a catalog for DocBook is install the DocBook stylesheets (see "Installing the DocBook Stylesheets" on page 492), which also sets up a catalog. You can also use the sample catalog from the schema distribution, which can be found at: http://docbook.org/xml/5.0/catalog.xml. The sample is a starting place, but it will almost surely not work "out of the box." To make it work, you need to make the catalog