This example builds on knowledge of Qt you should already have. If you take a look at the code of the example (and I mean its latest version, not the one that is two years old), you will notice there are practically just two lines of code that deal with MDI there. The example is so trivial, it doesn't need any explanation beyond what you should already know. Both those lines are explained in the documentation linked from the example. I do agree though that it looks silly that there is practically no text with the example. But it's still an example and not a tutorial.