Very often, when a musician talks about modulating or referring to a certain chord change modulates, is really applying/ understanding the concept of modulation? It is very common that from word of mouth or popular myths, a musician mistakenly learn a concept like this and that indefinitely (until it learns correctly), use it to express its ideas in a wrong way.
First and foremost, I would like to define the concepts/ techniques “Modulation” and “Modal Interchange” in order to have a starting point in comparison.
MODULATION: modulation is spoken when, through some of its techniques (prepared, not prepared, etc.), a cadence of chords results in “complety abandon the main key” (tonal center) or the one that is currently in use.
MODAL INTERCHANGE: Modal interchange is spoken when taking the same root or tonal center (Ex: C), the mode (scale) is changed to use the chords that derive from this change. This technique “does not leave the main key” (tonal center), but shares notes with the tonalities in course.
In Ex1, when passing from the first key to the second (C to E), the notes that reaffirm it are lost, ie Root (C) and Fifth (G), since in the tonality of E both notes are Altered (C # and G #).
In Ex2 both keys share the notes that reaffirm the first; That is, Root (C) and Fifth (G) that are still valid in the second key but as different degrees (C as 1st towards 2nd and G as 5th to 6th). This example is a modal exchange from C Ionian to C Dorian.
Both techniques represent a change of key but the modulation leaves the tonal center and the modal exchange does not. Like everything in art (and life!) None is better or worse, simply different and it is necessary to differentiate them to achieve a greater understanding and to play with greater freedom.
For those curious, here are some examples of modulation and modal interchange found in songs and will consist of an exercise to realize and differentiate techniques: Giant steps, Recordame, All the things you are, Blue bossa, How high the moon, Body and Soul, among others.
Until the next post.