A mirror chord, also called a mirror-image chord, is a chord obtained by intervallic reflection of a structure with respect to a reference note. If a chord is formed by a certain arrangement of ascending intervals, its mirror chord presents the same relationships arranged in the opposite orientation.
A basic example is given by the relationship between the major triad and the minor triad. The major triad C–E–G presents, in its root position, a major third followed by a minor third:
C–E = 4 semitones
E–G = 3 semitones
The minor triad, on the other hand, presents the opposite arrangement:
C–E♭ = 3 semitones
E♭–G = 4 semitones
In this sense, major and minor can be read as two mirror forms: not because they coincide as chords, but because they arrange in reverse order the two internal intervals that form the fifth. This idea is generally related to the concept of inversion in music theory, where intervals, chords, or melodies can be reversed or transformed according to an inversion relationship. In musical set theory, there are also notions such as inversional equivalence and inversional symmetry, used to describe pitch sets that remain recognizable through inversion operations.
In the Solaria system, the concept of mirror chord is used specifically to indicate the reflection of the intervallic form of a chord with respect to the root or reference note. For example, the sigma chord of C:
For example, the sigma chord of C:
C–D–G–B♭–B
corresponds to the structure:
0 2 7 10 11
The internal distances between the notes are:
2 – 5 – 3 – 1
The mirror chord is obtained by arranging these distances in reverse order:
1 – 3 – 5 – 2
From this derives the structure:
0 1 4 9 11
that is:
C–D♭–E–A–B
The two chords share the same spatial interval vector, but present a reflected internal arrangement.