I know there is a term for two diatonic chords that are a thrid apart (and thus sharing 2 or more notes), but I forgot it.

Examples :

  • Am and C
  • Gm7 and Bb
  • IV-vi
  • vi-I
  • etc.
  • If you meant Gm7 and Bb6, then those are inversions of each other. – Tim Feb 7 at 15:24
  • @Tim I guess Gm7 and Bb6 would fit my description, but I'm not looking for the term inversion, which does not always apply for these chords (see: Am and C) – 021 Feb 7 at 15:39
  • Things get a bit tricky when talking about relative chords because relative is really a term applied to keys. By extension it gets used to describe chord relationships. – Michael Curtis Feb 7 at 15:44
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    in German they are called "Terz verwandt". This means third-related. The scales and keys are called relative keys, but I would call them diatonic third-relations, while mediants seem to go under "chromatic third relation." jstor.org/stable/745872?seq=1 – Albrecht Hügli Feb 7 at 17:36

For those examples - where the chords are diatonic, belonging to one key - you can call them relative major and minor. Usually the usage is one chord referring to the other: "Am is the relative minor of C major" or "C major is the relative major of Am."

There is another kind of chord pairing where the roots are a third apart but not in the same key, and sharing the same chord quality: chromatic mediants. Very different from your example, but I you might want to be aware.

You didn't ask, but it also may be interesting to point out a few common progressions featuring relative chords:

  • in rock/pop the "doo wop" progression I vi IV V, or it's common to just vamp back and forth between I and vi.
  • Baroque and early classical often used roots by descending thirds like I vi IV ii... where the first two chord are relatives.
  • from the same time period there are two harmonic sequences that feature moves between relative pairs. Falling thirds in major I V6 sequenced to the relative minor vi iii6, the other one in minor doesn't have a name I know of but can be seen in examples like Handel's Sarabande in D Minor or the Passamezzo Antico ground bass where i V is then sequenced at the relative major III VII.

The reason I mention these examples is because root movement by third is often described as a weak progression. Obviously it isn't weak in the sense of ineffective when it is so commonly used. Weak in this sense more or less means root progression not by fifth. It does seem significant that these various progressions involve one of the relative chords being a tonic and then somewhere dominant harmony is used to make the tonality clear.

Strictly speaking "relative" is a descriptor for keys. From Grove's dictionary...

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...but people commonly use relative to describe chord relationships.

As the term applies to keys it's no surprise that when talking about chord relationships, relatives imply one or the other chord is a tonic, like I and vi in major keys or i and III in minor keys.

  • Yes, my question is in the case the chords are diatonic, I could add that to the question. I hoped to find a term less specific than relative maj/min, I feel like relative maj/min is only expressing the vi-I, where I'd like a term that would also work for IV-vi, V-vii°, etc... Have you heard the term "mediants chords" ? I feel like this could be the term I'm looking for. Thank you :) – 021 Feb 7 at 15:12
  • Based on the term "chromatic mediants", we could imagine it could be called "diatonic mediants" ? Or just "mediants", since diatonic is often implicit. It seems to be the same concept. – 021 Feb 7 at 15:14
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    Yeah, relative major/minor does imply (at least to me) one chord or the other is a tonic. I don't know of another more generic, but widely used term. "Root progression by third" would be the concise description. I don't think anyone would misconstrue that to include chromatic mediants which a special enough that people use the term whenever they are present. – Michael Curtis Feb 7 at 15:32
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    I thought I heard such a term in a video of the YouTube channel 12tone. But maybe I don't remember well, or maybe he used a not widely used term, or an incorrect one. Thank you for your hindsights, they're welcome ;) I'll leave the question open, if it happens someone would know such a more generic term to describe this. As for "Progression by third", it was not what I recall of it, but I guess it would fit the description ! – 021 Feb 7 at 15:37
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    I will add a note to my answer about why it applies best to tonic pairs. – Michael Curtis Feb 7 at 15:38

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