DocBook provides a system for writing structured documents using SGML or XML. It is particularly well-suited to books and papers about computer hardware and software, though it is by no means limited to them. DocBook is a document type definition (DTD). Because it is a large and robust DTD, and because its main structures correspond to the general notion of what constitutes a book, DocBook has been adopted by a large and growing community of authors. DocBook is supported “out of the box” by a number of commercial tools, and support for it is rapidly growing in a number of free software environments. In short, DocBook is an easy-to-understand and widely used DTD. Dozens of organizations use DocBook for millions of pages of documentation, in various print and online formats, worldwide.