Using the "?" character specifies that the element named is optional. Thus, in the following code snippet, we specify that every CONTACT must have a NAME and either a PHONE or EMAIL and may have an optional ADDRESS child.

	<!ELEMENT CONTACT (NAME, (PHONE | EMAIL), ADDRESS?)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT PHONE (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>
	<!ELEMENT ADDRESS (STREET+, CITY, STATE, ZIP, COUNTRY?)
	<!ELEMENT STREET (#PCDATA)>
	<!ELEMENT CITY (#PCDATA)>
	<!ELEMENT STATE (#PCDATA)>
	<!ELEMENT ZIP (#PCDATA)>
	<!ELEMENT COUNTRY (#PCDATA)>