base class undefined error

So I have a structure like this: //file1.h class a { ... } //file2.h #include "file1.h" class b : public a { .. } so everything works fine. BUT, when I introduce another class like this: //file3.h #include "file2.h" class c { ... } things get ugly and I get the compilation error: "error C2504: 'a' : base class undefined". Notice the 'a', wher the error is. What could be the problem? When I comment the line #include "file2.h" everything is fine again...