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I came across an unusual chord progression in an A minor track: iv-I.

Correct me if I'm wrong, but my first guess is that the I is borrowed from the major key. However, after thinking a while, I started to wonder why most chord progressions assume chords from either the major or natural minor scales (I would also like an explanation why that is).

Is it possible that the chords could be derived from different scales as well? For example, jazz musicians know a lot of scales under their belt. Could each of those scales form a different type of chord progressions?

I may be asking this question in a strange way, considering that I only know basic music theory. So please take that into account as there may be a better way to ask this.

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You will find that most chords can be derived from more than one scale - usually this will change the chord's function.

Consider the humble C major chord. It can be the Tonic (I) chord in C, the Subdominant (IV) in G and the Dominant (V) in F. The same is true for other major and minor chords. In other words, it's not enough to look at the root and type of the chord, we must also consider the surrounding chords to determine which scale/key the chord is derived from.

You specifically ask why chord progression assume major or minor scales. This isn't necessarily true. For example, the I-IV-bVII-I progression commonly found in rock songs is actually based on the Mixolydian mode, so G Mixolydian would give you G-C-F-G. Except, that G Mixolydian is a mode of C major, so if you really wanted to, you could bring it all back around to a major scale (it's not the sensible thing to do, because your "tonic" note in this case is G).

When we start looking at extended chords, we can see how they derive from particular modes. For example Cmaj7 can be viewed as a chord in either C major or C Lydian (G major), but G7 is unique to G Mixolydian (a C major mode) as are all further chords in the Dominant series.

Finally, it is possible to create highly clustered harmonies (usually very dissonant) implying a scale outside the minor/major system and its modes - possibly even pure chromaticism. It's not often done, because the results sound bad to our ears, but that is a cultural issue as much as a question of actual physics of sound (pretty much anything outside major/minor triads is dissonant to some extent).

To briefly address the resolution that led to this question, it is not that uncommon to introduce harmonic substitutions in order to achieve the desired effect and resolutions to a major tonic are often seen as more "satisfying" than resolutions to minor. Other things you're likely to encounter are V in a minor key (harmonic minor), IV and V in minor (melodic minor), the aforementioned substitution of bVII for V in major keys, and a whole host of others that may either change the quality of the key (as with the minor variants), imply multiple tonalities or even modulate to other keys. At the end of the day, what sounds "right" to the composer is what goes - theory may only catalogue such substitutions and attempt to explain them.

Major and minor keys form the basis of the Western musical system, so it's to be expected that most music will exploit these to a lesser or greater extent. At the same time, we've outgrown a lot of the once inviolable rules of harmony and quite a lot of modern music has departed from the old diatonic system entirely. Popular music styles like rock and jazz may largely work within the old system, but often bend it in interesting ways - if only because a lot of the songwriters never bothered to learn what they (supposedly) shouldn't be doing.

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If that is the cadence the piece ends on then it very well may be A Picardy third, Picardy cadence or Tierce Picarde. That is to say a Cadence in a minor key that ends on a Major Tonic chord.

It traditional harmony this is only done at the very end of a piece but it could be that some popular music just liked the sound and went with it.

Read more here.

  • More commonly - tierce de Picardie. – Tim Sep 7 '15 at 9:05
  • I was wondering about the 'de' myself. – Neil Meyer Sep 7 '15 at 9:15

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