Explain the concept of modal interchange and provide three examples of its application in a jazz standard.

Modal interchange, also known as modal mixture or borrowed chords, is a harmonic technique used in jazz and other genres where chords are "borrowed" from parallel modes or scales. Instead of solely relying on the diatonic chords within a single key, composers and improvisers incorporate chords from related modes that share the same root. This technique introduces color, expands harmonic possibilities, and creates a more sophisticated and nuanced harmonic texture. It's a way to move beyond the basic sound of a key without completely abandoning it. Modal interchange adds a sense of surprise and a rich palette of harmonic flavors that makes jazz harmony so appealing and complex. The beauty of it lies in its ability to subtly shift the emotional landscape within a piece.

Here are three examples of modal interchange within the context of jazz standards:

1. "All The Things You Are" – The bridge of this famous standard features several instances of modal interchange, primarily involving chords borrowed from the parallel minor and other related modes. For example, the bridge moves towards the relative minor key, often with a passing chord taken from the parallel minor. If the song is in Ab major, the bridge uses the parallel minor scale of Ab minor. The section uses a Bbm7 (minor 7th chord from the ii of Ab minor) instead of the diatonic ii chord (Bbm7). Also, an Eb7 chord is followed by an Ab maj7 chord. The Ab major chord is followed by the minor chord, Ab minor. This creates a rich movement using borrowed chords. The Bb diminished chord, for example, which is commonly found in the harmonic minor, is also used to transition into the C minor section, lending a sense of drama and tension to the harmony. These modal interchanges significantly enrich the standard.

2. "Autumn Leaves" – In the standard version of Autumn Leaves, a common modal interchange can be seen, especially as it progresses through the song. It’s known to begin in G major. But, towards the second part of the A section, a Bb major chord may be used, which is a borrowed chord from the parallel minor, G minor, and acts as a substitute for a subdominant chord. This use of a major chord in a typically minor context (a subdominant in the minor key would be minor) is a clear example of mode mixture. This borrowed chord adds a contrasting color to the otherwise diatonic progression. Furthermore, the song often includes a chromatic passing chord or passing chords, borrowed from harmonic minor scale, to lead into the minor chord in the A section. These chords add drama and tension, and would not be a standard part of major key harmonies, creating a sophisticated and emotional effect.

3. "Stella by Starlight" – This standard features various instances of modal interchange, contributing to its lush and sophisticated harmonic structure. A common example occurs with the use of a borrowed minor iv chord, often played as a minor 7 chord. For instance, if the main key is Bb major, a move from the diatonic ii chord (Cm7) to a borrowed Ebm7 chord (minor iv in Bb major, typically Eb maj7) provides a subtle shift in color by borrowing the iv chord from Bb minor. It adds a dramatic and melancholic touch. Also, instead of using standard dominant V chords, you can find flat ii7 chords or the flat iii7, borrowed from the melodic minor scale, that serves as dominant functioning chords that resolve to the tonic. These borrowed chords provide additional chromatic movement and color. The common use of borrowed dominant chords from the parallel minor further enriches the harmonic language.