A mirror chord, also called a specular 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 distances, its mirror chord presents the same relationships arranged in inverse orientation.
An elementary 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 presents instead 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 distances that form the fifth. This idea is related, in a general sense, to the concept of inversion in music theory, where intervals, chords or melodies can be inverted or transformed according to an inversion relationship. In the theory of musical sets there are also notions such as inversional equivalence and inversional symmetry, used to describe pitch sets that remain recognisable through inversion operations.
In the system of Solaria, the concept of mirror chord is used in a specific way 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:
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.