The Extensible Markup Language (XML) is a W3C-recommended general-purpose markup language. XML is considered "general-purpose" because it enables anyone to originate and use a markup language for many types of applications and problem domains. Numerous formally defined markup languages are based on XML, such as RSS, XHTML, SVG, XSLT and thousands of others. Specific to XML is the use of angle-brackets to identify data and metadata (elements and attributes). By leaving the names, allowable hierarchy, and meanings of the elements and attributes open and definable by a customizable schema, XML provides a syntactic foundation for the creation of custom, XML-based markup languages. In this way, XML contrasts with HTML, which has an inflexible, single-purpose vocabulary of elements and attributes that, in general, cannot be repurposed. XML is an open, fee-free standard.