The internal DTD subset is the portion of the document type definition included directly within the document type declaration between the [ and ] characters. The internal DTD subset can contain markup declarations and parameter entity references, but not conditional sections. A single document may have both internal and external DTD subsets, which, when taken together, form the complete document type definition. The following example shows an internal subset, which appears between the [ and ] characters:

<!DOCTYPE furniture_item SYSTEM "furniture.dtd"
[
<!ENTITY % bookcase_ex SYSTEM "Bookcase_ex.ent">
     
%bookcase_ex;
     
<!ENTITY bookcase_pic SYSTEM "bookcase.gif" NDATA gif>
<!ENTITY parts_list SYSTEM "parts_list.ent">
]>