Composers have found that using only the diatonic scale degrees is very limiting and often results in the music sounding static and dull. By adding chords that do not naturally occur within a particular major or minor key, music may become more expressive and have more variety. One of the most common functional non-diatonic harmonic practices involves intermixing chords from the parallel major or minor mode. Below is a chart of the most common diatonic chords in major and minor keys. As you see, many of the scale degrees have chords that have the same root, but a different chord quality. All of the circled pairs of chords may be interchanged.
When chords from the parallel major or minor are used, they are referred to as borrowed chords. In chords where the root of the chord is altered by an accidental, that same accidental is added before the roman numeral to indicate this alteration (i.e.: the borrowed VI chord above would be referred to as a "bVI" - read "flat six chord"). The most common use is when the music is in a major key, and chords from the minor mode are borrowed. This often creates a darker, "sadder" feel to the music, and is frequently used in vocal works to reflect a change in mood in the text. (a process called text painting.) the following are commonly found borrowed:
i - ii(dim) - bIII - iv - bVI
In the following example, the borrowed iv and bVI chords create a definite change in the mood of the phrase.
Because the use of a melodic minor scale results in IV, V, and diminished vii chords, these are not considered borrowed. The only frequently found borrowed chord in minor is the borrowed tonic triad, and this is commonly only found as the final chord of a work. In this particular usage, it is referred to as a Picardy Third chord. Composers of the common-practice period often ended works on a Picardy Third to create the effect of a "happy ending".
When a musical passage changes to it's parallel major or minor mode and remains in that new mode for an extended period of time, our ears may hear it not as a few borrowed chords, but a change of mode. When music changes mode in a more permanent manner, there is often a key change to indicate this. Changes of mode may be as short as 2-3 measures in a slow tempo, or much longer. In the first example, the music only changes from major to minor for a few borrowed chords. In the second example, it remains in the Major mode and is an actual change of mode. (The use of the C# and G# will be covered in the following Section)
Change of Mode:
Some of the signs that a passage has changed mode, as opposed to just borrowing from the other parallel mode, are: