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I was checking a video about this subject and came across a question: when I have a lower chord like C2 G Bb D and as mirror D F# A E what determines that the mirror chord starts in D when the next example is A1 E B C and as mirror we have E F C G ?

  • In both examples, it is as if there is a mirror extending vertically from the center of the Ab in between the chords. The other note on a keyboard with this type of visual symmetry is D. – The Chaz 2.0 Mar 10 '18 at 14:24
  • See also music.stackexchange.com/questions/56394/… – Richard Mar 10 '18 at 19:21
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The notion of "mirrored chords" is based around an earlier notion of an axis of symmetry. When we have a pitch (or pitches), we find its mirror image around a given axis of symmetry to find the resultant pitch.

Take, for instance, a C. If our axis of symmetry is the G above it, we see that our C is a perfect fifth below the axis. Therefore, its mirror image would be a perfect fifth above G, which would be a D. We say that C "inverts around G" to D.

(There's a distinction here between pitch inversion and pitch-class inversion, but that's for another time.)

An easy way to determine these inversions/mirror chords is with the twelve pitch classes arranged in a clock:

enter image description here

Now, we look for a mirrored interval. In your original example of C G Bf D, we see an ascending perfect fifth between C and G. To find the axis of symmetry, we look for a descending perfect fifth in the mirror chord: that between E and A. Now we have to find how C inverts to E and how G inverts to A.

Basically, we can just split the difference: find the spot halfway between C and E (D) and connect that to the spot halfway between G and A (A♭):

enter image description here

From this, we see that the axis of symmetry is D/A♭.

Now we can doublecheck your second example of A E B C. A inverts to G, E inverts to C, B inverts to F, and C inverts to E, which gives your collection of E F C G.


Note that not all "mirror chords" use this pitch axis; different pieces use different axes based on various compositional decisions.

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