A chord is ambiguous when it can be associated or implies more than one tonality or mode. As an example, every major or minor triad can be associated to two or more tonalities. For instance, the C major chord exists in C major, G major and F major tonalities; or a Dm belongs to C major, F major or Bb major tonalities, and so on.
The fact that a chord may have multiple meanings or associations, introduces the possibility of functional ambiguity. If you have a C major chord and the melody also starts in C major tonality but then, at some point you use an F# or a Bb (or both) in your melody, then the ambiguity is introduced because the listener can’t associate the C major chord to the C major tonality, exclusively.
Probably, the most obvious cases for harmonic ambiguity are the diminished seventh chord and the augmented triad. By themselves, these chords aren’t very useful at expressing a tonal center and their dissonant character can be tamed by making simple chord transformations that in turn will point to different tonal centers:
Some of the possible chord transformations of a C+ chord by moving only one of the chord voicings, opening up several modulation possibilities
This ambiguity occurs mainly because these chords are built symmetrically by dividing the octave in equal parts. Not only they may point to different tonalities but their root is not defined.
As seen in previous posts, the diminished seventh chord can be transformed into four different dominant chords that can be used to establish the tonic of four major and four minor tonalities – eight different tonal centers, in total. The diminished chord can be introduced by chromatic alteration of the fifth in a minor chord, while the augmented chord can be introduced by chromatic alteration of the fifth in a major chord.
In the case of the latter, any chord tone can be used as a leading tone that resolves to the next chord; it can be used as an altered dominant or a secondary dominant chord; and it can also introduce the new tonic, by lowering the fifth:
There are other chord formations that can induce ambiguity, such as chords built by fourths, fifths or suspended chords. More often than not, this ambiguity is introduced by the lack of the third in relation to the root of that chord, meaning that the composer is free to use major and/or minor modes on top of the harmonic content; and because there is no clear root, these chords don’t have an impetus to resolve to any particular chord.
Even more so, the chord quality can be redefined depending on the existence of a bass note that may confirm, or not, any of the chord tones, and thus implying a different chord altogether:
Same quartal chord with different bass root
Examples of this type of harmonic ambiguity can be found in famous chord formations, like the Tristan chord, from Wagner, or the Mystic chord from Scriabin. Such chords, if expanded, may contain all the twelve tones and because of that, those can easily be used in a multitude of tonal and atonal contexts.
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