While changing key is easy with the basic Movable Do Disc, we gain even more insight in key changes by combining it with the larger MDD (with or without the APA), showing us two tonal layers simultaneously.
We can see and exactly navigate any key change: not only can we easily see the number of changes, but it is clear exactly what the consequences of the key change are, with pitches change function or are being altered to fit in the new key.
With the inner MDD as the starting key, this image shows a movement of “-3” through the circle of fifths.
We can see that the the new key (the outer MDD) shares four pitches with the old key (the inner MDD), and has three new pitches in the diatonic scale, that are the result of lowering the tones ti, mi, and la of the old key. On the Key Relationships sheet, you can see which relative tones will represent the new pitches in the destination key.
This is true of any move of -3, whether it be from C major to C minor, from D major to F major, or from F# minor to A minor.
Changing key is easiest through downward or upward leading tones (such as fa or ti of the new key, or si in the case of a minor key), but can be done through any tone!
The Key Relationships sheet shows all possible relationships between different keys, with their distinguishing tones.
It enables you to quickly analyze what type of move you are dealing with, and shows you that moving from Db to Ab for example, is the same relative movement as going from C to G.
Moves to the left (-) are the colour of fa, because that is the distinguishing and “flattest” tone in that direction, while moves to the right (+) have been given the colour of ti, the “sharpest” tone of the new key.
Taking this Circle of Fifths and placing the starting key on “0” on top of the Key Relationships sheet shows the relativity of all key changes: any key can be taken as the starting point, just as we encounter it in “real” music.
The pitch names on the Circle of Fifths are in red, because they show which tone is do. The lines of the staff are coloured with the colours of the diatonic scale tones they represent as on the Movable Do Disc.
Notice how the last flat is always on the fa line or in the fa space, while the last sharp is always on the ti line or in the ti space. A change of key also means a change of the functions of the five lines of the staff!