“Tonicization” is the treatment of a non-tonic chord if it were, temporarily, the tonic. There are degrees of tonicization, ranging from less substantial to more substantial ones. On the continuum shown below, a secondary dominant is a less substantive tonicization, as it involves just a dominant chord and its resolution. Modulation is more substantial, as it indicates a change of tonic, usually with a cadence in the new key. In between these two extremes is a “tonal area,” a tonicization that involves several chords, usually a pre-dominant–dominant–tonic progression in the new key, but with no cadence in it.
Degrees of Tonicization: A Continuum with Tonal Area in the Middle
Secondary dominant <—————– Tonal area ——————> Modulation
Here is a video on Tonal Areas:
Scores for the two songs discussed are provided below.
Richard Rodgers’s Blue Moon (start at the bridge)
Richard Rodgers’s You Are Too Beautiful (start at the bridge, which begins on page 3, the last three measures.