Hello I just read the first book of hooktheory but there is something I can quite understand, what about the III major chord, the II major chord, the VI major chord and the VII major chord. For exemple I am currently re writing a track from avicii and in the chord progression there is a III major and a VII major, did I miss something ?
or does the first book only covers the major scale ?
Yeah the second book goes heavily into the function of these chords that use scale degrees outside the major scale. Specifically it covers secondary dominants and chords borrowed from other modes.
Ok thanks, but I mean is the rules of the first book also applies when working with songs written in minor keys ?
For example for a song written in C#minor, do we have the same chords I,ii,iii,IV,V,vi,vii ? And is the first chord 1-3-5, the second 2-4-6 etc. etc. ?
Technically, yes the chords you listed are there, but they have different notation/names. This is due to the minor scale being a major scale, but with the root being a 6th up, which also means b3, b6, and b7 scale degrees relative to the major scale.
For all minor scales, the chords are i ii0 III iv v VI VII. If you write it based in a major key, it would be vi vii0 I ii iii IV V. Note how in the major key the root starts on the 6th scale degree.
The changes between, say A major and A minor is 1-2-b3-4-5-b6-b7 (A-B-C#-D-E-F#-G# vs A-B-C-D-E-F-G).
So yes, there is the 1-3-5. 2-4-6, etc. structure, but it is based on the scale degrees of a minor scale instead of a major scale. So in A minor the triads would be ACE, BDF, etc.