>I am confused about how the include file works when using classes. I create a form class and specifiy an include file. I then subclass that class, the include file is no longer filled in but it appears the defines are still available.

>Can I use the include file again on the subclass without problems?

sure

Then I make a form from the subclass. Here again the include file is still not shown.
> Are the include files additive in nature?

Partially.

> I have no idea on how that works. It's not like the include file is a property of the class. Any tips?

VFP don't support include files as a Project/OOP property

you can use _INCLUDE, but it is not flexible.