There are two fundamental forms of modulation - between different tonal centres, and between different tonal types (major and minor tonalities). This means that we can modulate from the major tonality of C to:
- a different tonal centre, but the same tonal type, such as G major;
- the same tonal centre, but a different tonal type, which is c minor;
- a different tonal centre and a different tonal type, such as f minor.
A modulation can be between closely related or a distantly related keys, and it can be articulated in a manner which either smooths this transition or which highlights it as a sudden shift. The choice is the composer's, and in this section I will describe the relationships between the different tonalities and the methods used to move between them.
There are two important ways to measure the distance between any two keys. The first is the number of notes that they have in common - the more they share, the more similar they are; the second is the inherent similarity of any two keys which share the same tonic note - the only two keys which share the same tonic note are parallel major and minor keys.
Modulating between any two keys which are closely related is likely to be much less disruptive than direct modulation between any two distantly related keys. Of course, it may be desired for the modulation to be heard as a sudden and unexpected transition, in which case no preparation need be made, but if we wish to modulate smoothly to a distantly related key we can do this most effectively by using a series of closely related keys as stepping stones to the more distant key.