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XML is becoming a standard for data communication over the Internet. Like HTML, it is a markup language, but it supports a richer set of features, such as user-defined tags that allow both data and descriptive information about data to be represented within a single document. At the same time, presentation aspects remain decoupled from data representation. XML's flexibility lets it serve as a metalanguage for defining other markup languages specialized for specific contexts. A document type definition (DTD) describes the tags documents can use, customized to the specific semantic requirements of the application context, and the rules connecting tags with their contents. These capabilities make XML a common data format for data interchange between computer systems and between applications. XML's proliferation raises the question of how data transferred by XML documents can be read, stored, and queried. In other words, how can database management systems (DBMSs) handle XML documents?.