I’m just starting to learn music theory. I had a question: In Hook theory I - section 1.1-1.2 - we are shown the major scales without explanation as to why certain notes are part of that scale.
Given a note, say, D…I can figure out D major chord has D F# A by counting keys D …4th key from D is F#…3rd key from F# is A. So by doing that, I can tell that in D major scale has the notes D F# A. However, how do I know that the note E, G, B, C# also exist in the scale? Do I need to memorize that or is there a logical, step by step approach to discovering other notes in a scale?