Implementing XML Includes in DTD

Hi all.

Hopefully this is an easy question to answer.

I’m building an article/publication based website and was planning on storing most of the content for the site in XML (for manageability, automation and sharing purposes). I’m currently designing the layout for the XML files and I want to create a DTD (or XSD) to describe the various types of XML files we will be using throughout the site. In doing this I’ve begun to realize that several elements are actually being reused across various XML documents. My question is this:

Is there a way to include elements in a DTD (or XSD) that are defined in another file? It appears as if the w3 is doing this with XHTML using “mod” files, but I’m unfamiliar with the format, and (unfortunately) not an XML expert. Is it possible to define single or related elements in their own files and then include them as needed in various DTDs through some sort of “include statement”? Does XML Schema support this as well? Are their any problems with laying out my DTDs (or XSDs) in this “include” style of design?

Thanks,

Darren