What is a DTD?

• A DTD lists the elements, attributes, and entities contained in a document

• A DTD for songs:

  <!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                   LENGTH?, YEAR?, ARTIST+)>
  
  <!ELEMENT TITLE (#PCDATA)>
  
  <!ELEMENT COMPOSER (#PCDATA)>
  <!ELEMENT PRODUCER (#PCDATA)>
  <!ELEMENT PUBLISHER (#PCDATA)>
  <!ELEMENT LENGTH (#PCDATA)>
  <!-- This should be a four digit year like "1999",
       not a two-digit year like "99" -->
  <!ELEMENT YEAR (#PCDATA)>
  
  <!ELEMENT ARTIST (#PCDATA)>

• Normally stored in a separate file