Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form.  XML's design goals emphasize simplicity, generality, and usability over the Internet.  It is a textual data format with strong support for the languages of the world. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.  As of 2009, hundreds of XML-based languages have been developed, and XML-based formats have become the default for most office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork.