This is an example of using both an external DTD and an internal one for the same XML document. The external DTD resides in "tutorials.dtd" and is called first in the DOCTYPE declaration. The internal DTD follows the external one but still resides within the DOCTYPE declaration:
Here, I've added a new element called "summary". This element must be present under the "tutorial" element. Because this element hasn't been defined in the external DTD, I need to define it internally. Once again, we're setting the "standalone" attribute to "no" because we rely on an external resource.