Secrets of Chord Substitutions Revealed

The students of harmony must understand something very simple:

Different chords have common notes between them, an example is C major chord triad that has two common notes with its relative A minor, so it is so easy to modulate to it's relative, therefore, there are more common notes between the two tonal fields, but if you think in four notes, the 'A minor 7' will have all the notes that compose up the triad of its relative 'C major'.

So it is to play with common notes with other notes in the bass.
Who used first in music history, became a reference for others, as in the case of Chopin, Brahms, Fauré, Debussy, Ravel and Scriabin, that will give much ready ideas for the jazz players.

Then, in 'C major tonality', when the chord V has the diminished fifth (Vb5 or G(b5)), the harmonic function remains the same, it is not that the chord V has been replaced by 'Db', it is that the '5b' seems to have built a chord of 'Db7 (5+)' that are the same notes of the chord V79, then if you want in that chord V, drop the '9b', it will have the same face as the 'Db7', simply a coincidence, from the V chord, the same incidence of the coin falling on the same side, face or crown.

All this is allowed in Harmony only in the V chord funtion, who is the owner of the land, he is a contraventor, an armed bandit that tries to destabilize all harmonious sonorous expectancy, proving changes without authorization, that is, he who dominates in the area.

The other cases are chords with the add '6th', who will fall on the bass, giving an a idea that another chord is born, but having the same common notes of it's relative, than you should notice that it has fall a minor 3dr down, just like relatives major an minor happens.