Just as changes of harmony and changes of mode create a sense of motion and interest in music, changes in tonal centers create a sense of movement and interest in a larger sense. It is extremely difficult to find a work in the standard repertoire that functions in the same key throughout. Music that changes keys and establishes a new pitch (other than scale degree 1) as tonic, is said to have modulated. When a composition briefly (i.e. only one or two chords) emphasizes a new key by use of chords from that key, it is said to tonicize that key.
As we discussed in SECTION 13: Basic Harmonic Function, The V-I progression is one of the strongest functions in tonal music. A simple I-V-I progression can quickly establish a tonal center (tonic). By adding a seventh to the dominant chord, this function is further strengthened. Listen to the following three examples, and after each tonic-dominant-tonic progression, hum tonic:
Aural Examples (try to hum tonic after each chord progression):
I - V - I
i - V - i
I - V7 - I
Composers have found that by preceding any chord other than tonic with it's dominant (i.e. the V chord from the key of that chord), there is a brief emphasis on that chord and a sense of harmonic movement. In the following example in C Major, the ii chord (d minor) is preceded with a
The leading tone is the strongest tendency tone in a key. This tendency tone is present in the dominant V chord, and is largely responsible for it's strong harmonic function. The seventh of a V7 chord, scale degree 4, is also a strong tendency tone. When we use a secondary dominant chord, the added accidentals create new tendency chords that need to resolve in the same manner as diatonic ones. So the voice leading for a secondary dominant chord is the same as for a diatonic dominant chord: the "new" leading tone, i.e. the third of the chord, should resolve up by step (especially when in an outer voice), and the seventh of the chord, if present, should resolve down by step. Observe the voice leading in the following example. Notice also that because each of the tonicizations is brief, one never looses sense of "G" as tonic.
The vii-diminished chord also can substitute for the dominant chord in a tonicization, just as it does in diatonic music. This is also a method of tonicizing a chord, since it is still accomplishing the same result: an emphasis of a new tonic. Notice that when minor triads are tonicized, they are normally preceded with the form of vii chord that is from minor keys (a vii fully diminished seventh chord), whereas major triads are commonly preceded with the form of vii chord from major keys (a vii half-diminished seventh chord).
(Note: when the V chord is tonicized, it is frequently preceded by a I 6/4 chord which then resolves to the dominant. As noted in SECTION 16: Writing Chords in Inversion, this cadential 6/4 chord is often viewed as an extension of a V chord. Thus, this example does indeed "resolve properly", even though there appears to be a tonic chord between the secondary seventh chord and the dominant chord.)
Composers may emphasize a new tonal area by means of chords other than V and vii. In the following example, Chopin uses an extended tonicization in the second and fourth measures. When several chords all tonicize the same chord, they are usually all notated above a single line, as shown below. Notice how the Ab chord on the downbeat of measure 2 is normally labeled as simply a VI chord in c minor, but in the larger sense is the I chord of the whole Ab Major tonicization in that measure. As you listen to this example, you may hear these measures as a modulations (which is the topic of SECTION 22: Modulation). This is often the case in extended tonicizations, and is not incorrect either. It is mostly a matter of personal perception, and is one of the wonderful aspects that makes this such an enduring work in the literature.