Modulation is the process of moving from one tonality to another. Simple modulation is movement to a closely related key. Closely related keys are those adjacent to a key on the circle of fifths and their relative keys. Therefore, any one key has five closely related keys.
FIGURE 12.1. Closely Related Keys
Terms applied to keys in modulation are original key, old key, and new key. The original key is the beginning key. Usually, music begins and ends in the original key. In a modulation, the music moves from the old key to the new key retaining the key signature of the original key. When a series of modulations has taken the music far from the original key, sufficient phrases must remain to work back to the original key.
FIGURE 12.2. Modulation Series in a Composition
A pivot chord is used to effect a modulation. In simple modulation a common chord (diatonic in the old and new keys) is used as a pivot chord. For example, the tonic chord in C major could be used as the pivot chord to modulate to G major as the subdominant chord. Pivot chords are analyzed in both the old and new keys. The pivot chord should not be the dominant chord of the new key.
FIGURE 12.3. Pivot Chord Modulation
Accidentals used to create notes in a new key need not be indicated in figured bass when
accompanied with analysis.
FIGURE 12.4. Figuring of Accidentals in a Modulation