II
                Harmony Type: Pre-Dominant
                Scale Degrees Supported: 2 - 4 - 6
                Major Version: Minor
                Minor Version: Diminished
                Can Lead To: Dominant and other pre-dominant chords.
                Rules:
                    -- ii is the V of V and therefore intensifies the authentic cadence ii-V-I.
                    -- In minor the root position is rarely used, because of its diminished quality, precautions must be used if this
                        inversion is used.
                    -- First inversion ii chords can freely substitute for root inversion IV chords.
 

                III
                Harmony Type: Secondary Dominant
                Scale Degrees Supported: 3 - 5 - 7
                Major Version: Minor
                Minor Version: Major
                Can Lead To: VI mostly, other cases can apply.
                Rules:
                    -- In minor the major version is always used and is usually used the I-III-V progression, III being the mediant
                        between I and V.
                    -- III in minor is its relative major, therefore care must be taken when dealing with this chord for it can easily
                        slip into  the relative major key.
                    -- The 5th of the chord should never be doubled, seventh scale degree.

                IV
                Harmony Type: Pre-Dominant
                Scale Degrees Supported: 4 - 6 - 8
                Major Version: Major
                Minor Version: Minor
                Can Lead To: Dominant and other pre-dominant. In some situations it can lead to tonic.
                Rules:
                    -- IV-I is a plagel cadence.
                    -- First inversion IV chords can freely substitute for root inversion VI chords.
                    --IV⁶-V in minor only is a phrygian cadence.
                    -- IV⁶ can be also substitute for VI in deceptive cadences.

                V
                Harmony Type: Dominant
                Scale Degrees Supported: 5 - 7 - 2
                Major Version: Major
                Minor Version: Major
                Can Lead To: Tonic.
                Rules:
                    -- the 3rd of the chord, seventh scale degree, should never be doubled.
                    -- V-I is an authentic cadence.

                VI
                Harmony Type: Pre-Dominant
                Scale Degrees Supported: 6 - 1 - 3
                Major Version: Minor
                Minor Version: Major or Diminished
                Can Lead To: Dominant and other pre-dominant chords.
                Rules:
                    -- VI in major is the relative minor.
                    -- VI can freely substitute for I, usually with the 3rd of the chord doubled.
                    -- V-VI is a deceptive cadence.

                VII
                Harmony Type:  Secondary Dominant
                Scale Degrees Supported: 7 - 2 - 4
                Major Version: Diminished
                Minor Version: Major
                Can Lead To: III mostly, other cases can apply.
                Rules:
                    -- In major the root position is rarely used do to its diminished quality and precautions must be used if it is used in
                        root position.
                    -- Never double the root of VII chords.
                    -- VII in minor usually leads to III, VII being the V/III.
                    -- in major the vii^o6 can be used to expand the tonic chord.

                Progressions
                -- all chord progressions used during the common practice period follow the hierarchical system.