A Document Type Definition (DTD) is a specification file that contains a set of markup declarations that define a document type for an SGML-family markup language such as GML, SGML, XML, or HTML. The DTD specification file can be used to validate documents and defines the valid building blocks of an XML document, including a list of validated elements and attributes. A DTD can be declared inline inside an XML document or as an external reference. The DOCTYPE declaration appears in the syntactic fragment doctypedecl near the start of an XML document and establishes that the document is an instance of the type defined by the referenced DTD. The purpose of a DTD is to define the structure and the legal elements and attributes of an XML document. With a DTD, independent groups of people can agree to use a standard DTD for interchanging data, and an application can use a DTD to verify that XML data is valid.