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In the
Drop 3
voicing, the third highest note is dropped down an octave.
The
Open
voicing is a variation on the
Drop 2
voicing. One other chord
tone of the inversion is shifted by a further octave to add more musically
relevant distance between the notes of the chord.
This results in more distance between the chord tones, allowing the total
character of the chord to be less cluttered and dense.
Compositionally, musical accompaniments will often make use of
spread out voicings as they leave more ‘space’ for a melody.