Reminder: Role of DTD-s in XML

  <?xml version="1.0"?>
  <Staff>
    <Employee>
      <Name>
        <Surname>Herman</Surname>
        <Firstname>Ivan</Firstname>
      </Name>
      <Title>Dr</Title>
      <Dept>INS</Dept>
      <Started>01.10.88</Started>
      <Email>Ivan.Herman@cwi.nl</Email>
      <Tel>4163</Tel>
      <Fax>4199</Fax>
      <Building>Secondary</Building>
      <Room>C112</Room>
    </Employee>
  </Staff>

In traditional XML, possible tags are defined by a DTD (Document Type Definition)