The falling third from I to vi is often found at the beginning of major key music, particularly Bach chorales. The effect of the example below is of a further descending third between the strong first and third beats of the bar - vi to IV. The progression I - vi is the opening of very common progression in tonal music from Bach to the Beatles: I - vi - ii - V - I - a descending third followed by three descending fifths.
The second example shows the same progression of two descending third - this time without any interpolated chords. The i - VI opening is not so common in minor keys as it is in the major, at least as far as Bach is concerned.
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