Some DTD Rules (taken from w3tutorial)

<!-- this is an XML comment (same as HTML comments) -->
<!-- element has k child elements -->
<!Element element_name (child_element_name1, child_element_name2, ..., 
	child_element_namek)>  
<!-- one child element -->
<!Element element_name (child_element_name)> 
<!-- one or more child elements -->
<!Element element_name (child_element_name+)>
<!-- zero or more child elements -->
<!Element element_name (child_element_name*)> 
<!-- zero or one child 	elements -->
<!Element element_name (child_element_name?)> 
<!-- child_element1 or child_element2 (can be as many alternatives as
	needed) -->
<!Element element_name (child_element_name1|child_element_name2)> 


DTD EXAMPLES:
<!Element note (to,from,heading,body)> <!-- all child fields of note-->
<!Element note (message)>  <!-- only one message per note-->

More DTD Rules

<!ATTLIST element_name att_name att_type default_value>
<!-- value is character data -->
<!ATTLIST element_name att_name att_type CDATA>  
<!-- value is entity_name -->
<!ATTLIST element_name att_name att_type ENTITY> 
<!-- value is unique id -->
<!ATTLIST element_name att_name att_type ID>
<!-- value is unique id -->
<!ATTLIST element_name att_name att_type ID> 
<!-- forces user to  define value for this attribute, but there isn't 
a default value-->
<!ATTLIST element_name att_name att_type #REQUIRED>


EXAMPLES:
#1
DTD example:
<!ATTLIST payment type CDATA "check">
sample XML described:
<payment type="check" />

#2
DTD example:
<!-- square element is empty -->
<!ELEMENT square EMPTY>  
<!-- default value is "0" but can be character data that will not be parsed-->
<!ATTLIST square width CDATA "0"> 
sample XML described:
<square width="100" />

DTD:
<!-- string is value of entity; used  by "& entity_name" notation -->
<!ENTITY entity_name "entity_value">  


EXAMPLES:
DTD Example:
<!ENTITY writer "Donald Duck.">
sample XML described:
<author>&writer;</author>