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.