Purposely I won't talk about modes, not in this “layer” of lessons, which have no focus on modality in general, except for those modes that are, in fact, the major and minor scales. Nonetheless, in the previous lesson I've dropped clues about the existence of modes and gave also a couple of pointers. My plan doesn't include modality in depth, though maybe I could have listed the diatonic modes, at least. The correct place should have been the previous lesson. I could resume the argument when and if I will talk about musical genres like jazz, or put them in an addendum/appendix to complete that part of Lesson 4 where I mentioned the Dorian mode; I could also list them in the next lesson, but this was on intervals, so I need an excuse to be back on that topic. Thinking about it…

Thank you for the feedback and the suggestions!