A DTD lists the elements, attributes, and entities contained in a document
A DTD defines the relationships between different elements and attributes
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