But in my opinion, how the notes that you play simultaneously sound together is the core of music theory. A good chord progression can feel like a journey and it will set the mood of the entire song. Therefore, I would argue that note frequencies and the reasons why certain tuning systems are preferred in certain situations are the key concept to understand if one wants to elicit a chosen mood in the listener.
But in my opinion, how the notes that you play simultaneously sound together is the core of music theory. A good chord progression can feel like a journey and it will set the mood of the entire song. Therefore, I would argue that note frequencies and the reasons why certain tuning systems are preferred in certain situations are the key concept to understand if one wants to elicit a chosen mood in the listener.