Functional Harmonic Progression
I (tonic chord) can move to any function
The strongest progressions move from one level of the diagram to the one immediately below
A weaker progression skips a level (like vi to V, or ii to I), and usually needs the strong movement of lines to compensate.
Skipping two levels (as in vi to I or iii to V) is very weak, and has little feeling of chord change
Moving upward in the diagram from any chord except I is a retrogression; V to vi is a deceptive cadence
General guidelines in harmonization
Use mostly strong progressions
Use weaker progressions sparingly, on weak beats, or with strong movement of lines, and never leading toward cadences
Cadences should follow familiar patterns (half, authentic, plagal, or deceptive)
Bass lines can be smoothed our through the use of 1st inversion chords
2nd inversion triads, other than the cadential tonic six-four, should be used sparingly, and almost never on strong beats (again, the exception being the cadential tonic six-four.)
Don’t repeat a chord or bass note from a weak beat to a strong.