Harmony: Interchange in Mode

The movement from the major triad to its parallel minor and vice versa by raising or lowering the third is used to cause a modulation called interchange in mode. If the music stays in the major and its corresponding parallel minor for a bit and goes back and forth, this can add great interest to the melodic line of the music. A very good example comes to mind as the first Nachmusic in Mahler's Seventh Symphony, but it is common enough in popular music as well.

This is suggested in the harmony of the scale degrees by the possibility that any triad can switch from major to triad or vice versa. Certiantly I can go to its corresponding minor triad, so in C-major, the Tonic triad or I, the C-Major Triad can go to the C-Minor triad by flattening its third: E to E-flat. Similarly, we noted that iii, E-minor, can become E-Major by raising the G to G# and modulate to iv. Similarly, the relative minor chord can become a major chord, A-minor to A-Major by raising the C to C#. In fact, the V/vi vi or V/vi to VI is an interchange of mode if we stay a bit in the new key. The ambiguity of major and minor suggested by V/vi is a very important innovation in the history of harmony.