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.
- Am and C
- Gm7 and Bb
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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
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:
I vi IV V, or it's common to just vamp back and forth between
I vi IV ii...where the first two chord are relatives.
I V6sequenced 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 Vis then sequenced at the relative major
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...
...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
vi in major keys or
III in minor keys.