Following Volume 1, Volume 2 explains music theory in a total of 21 topics.
This is suitable for those who want to improve their playing skills or learn music theory in more depth.
--- p.7

Major and minor chords are triads made up of the root, third, and fifth notes.
The difference between two chords is determined by the interval between the root and the third note.
Both major and minor chords have the same interval of a perfect fifth between the root and the fifth note, but different intervals between the root and the third note.
--- p.12

In Diminished Chord, Diminish means 'to decrease, to reduce'.
Therefore, diminished chords can be interpreted as 'reduced chords', and are therefore also called diminished triads.
So, what has been reduced? Let's compare it to major and minor chords.
If you lower the third note of the C chord by a semitone, you get the Cm chord.
If you lower the 5th note here by a semitone, this chord is called a diminished chord.
--- p.26

As the word 'substitute' suggests, a substitute chord is a chord that replaces a major triad.
It might be easier to understand if you think of it as a stand-in for the main character in a movie.
We studied the major triads corresponding to the 1st, 4th, and 5th notes above.
But if you only use these three chords, the song can become too monotonous.
To break this monotony, we use proxy code.
The surrogate chords are the four chords other than the major triad.

--- p.100