Explain the concept of modal interchange and demonstrate how it can be used to create harmonic variety and color within a tonal composition, giving specific examples with chord symbols and analysis.

Modal interchange, also known as mode mixture or borrowed chords, is a harmonic technique used in tonal music where chords are "borrowed" from a parallel key or mode to add color, variety, and expressive depth to a composition. It involves using chords that are not diatonic to the primary key of the piece, but instead derive from related modes like the parallel minor, the Lydian mode, or the Phrygian mode. The key here is that the root remains the same, but its quality changes based on the other modes. By introducing these borrowed chords, composers can create moments of unexpected harmonic shifts, darker or brighter tonal color, or a sense of nostalgia or exoticism. The most common form of modal interchange involves borrowing chords from the parallel minor key. For instance, in C major, the parallel minor is C minor. From C minor, we might borrow the submediant (♭VI), which would be A♭ major (Ab major). This chord is not diatonic to C major, where the submediant is A minor. Another very common borrowed chord from the parallel minor is the subdominant (iv), which would be F minor in C major. In the major key, the subdominant is F major. Here are some examples of modal interchange in practice: 1. Borrowed ♭VI (Submediant from Minor): ....