Modal interchange means borrowing chords from a parallel mode. Many significant such modal interchanges are widely used and hence quite popular among composers and songwriters. Both Andalusian cadence and Neapolitan 2 are examples of borrowing chords from the Phrygian mode. Given that Phrygian is a minor mode, we often see these 2 examples in songs written in minor keys. In a minor key the chords are i iio III iv v VI and VII, with one four, and five being minor chords, three six, and seven as major, and two being diminished.
In a Phrygian mode, two becomes a flat major and five is a major instead of a minor. So if there is a song written in the minor key, but five is played as a major instead of minor, that modal interchange is also called the Andalusian cadence, something we hear in flamenco a lot. The other example is playing a flat major two instead of a diminished two. This modal interchange is called Neapolitan 2. In Neapolitan 2, often that flat major two is followed by a five dominant 7, back to one. Let’s say a song is written in the key of Am. If a progression goes like Am Bb E7 Am, that Bb chord would be a demonstration of the Neapolitan 2 followed by 5 dominant 7th and back to Am. This approach is used if the goal is to enhance the sadness in the tune.
