The following DTD is a sample order DTD:
<!ELEMENT CustomerID EMPTY> <!ATTLIST CustomerID number CDATA #REQUIRED name CDATA #REQUIRED> <!ELEMENT Customer (CustomerID, Address, (Contact, Phone)? ) > <!ELEMENT Address (Street,City,State,PostalCode,Country) > <!ELEMENT Street (#PCDATA) > <!ELEMENT City (#PCDATA) > <!ELEMENT State (#PCDATA) > <!ELEMENT PostalCode (#PCDATA) > <!ELEMENT Counry (#PCDATA) > <!ELEMENT Contact (#PCDATA) > <!ELEMENT Phone (#PCDATA) > <!ELEMENT Order (Customer, Order.date, LineItems) > <!ATTLIST Order number CDATA #REQUIRED status (Open | BackOrdered | Shipped | Closed) "Open" > <!ELEMENT Order.date (#PCDATA) > <!ELEMENT LineItems (LineItem+) > <!ELEMENT LineItem (Quantity, Item) > <!ATTLIST LineItem number CDATA #REQUIRED > <!ELEMENT Quantity (#PCDATA) > <!ELEMENT Item (Description, UnitPrice) > <!ATTLIST Item number CDATA #REQUIRED > <!ELEMENT Description (#PCDATA) > <!ELEMENT UnitPrice (#PCDATA) >