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# DTD Introduction * * * A Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements. A DTD can be declared inline inside an XML document, or as an external reference. * * * ## Internal DOCTYPE Declaration If the DTD is included in your XML source file, it should be wrapped within a DOCTYPE declaration with the following syntax: An XML document with an internal DTD (please open in IE5 or higher and select "View Source"): <!DOCTYPE note > Tove Jani Reminder Don't forget me this weekend [Open this XML file in your browser and select the "View Source" command.](#) The DTD above is explained as: * **!DOCTYPE note** (line 2) defines that this document is of type **note**. * **!ELEMENT note** (line 3) defines the **note** element to have four elements: "to, from, heading, body" * **!ELEMENT to** (line 4) defines the **to** element to be of type "#PCDATA" * **!ELEMENT from** (line 5) defines the **from** element to be of type "#PCDATA" * **!ELEMENT heading** (line 6) defines the **heading** element to be of type "#PCDATA" * **!ELEMENT body** (line 7) defines the **body** element to be of type "#PCDATA" * * * ## External Document Declaration If the DTD is outside the XML source file, it should be wrapped within a DOCTYPE definition with the following syntax: This XML document is the same as the XML document above, but it has an external DTD: ((#), and select the "View Source" command.) Tove Jani Reminder Don't forget me this weekend! This is the "note.dtd" file containing the DTD: * * * ## Why Use DTD? With a DTD, each of your XML files can carry a description of its own format. With a DTD, independent groups of people can agree on a standard DTD for exchanging data. Your application can use a standard DTD to verify data received from an external source. You can also use a DTD to verify your own data.