Tonicization: The tonic pitch and tonic chord are established by context. For example, the V -> I progression is a tonicizing progression because of the strong harmonic closure of the downward fifth. In addition, V invariably contains the leading tone. The ascending half-step movement of the leading tone provides melodic focus on the tonic pitch. This complements and reinforces the harmonic closure produced by the roots of the progression.
Harmonic Prolongation: Likewise, any note can be tonicized if it is the goal of fifth relation cycles (i.e. I-V-I, vi-ii-vi, and so on). The effect of these tonicizing cycles is to prolong a harmony other than the tonic chord. Focus remains on the root of this chord as long as it remains the root of fifth relation progressions. This is an important variety technique as it allows focus to shift to other notes in a key and to the chord colors that are related to these notes.
The harmonic focus is less intense if the root of the progression is not preceded by a half-step, a leading tone-like movement.Example 5: Tonicization And Harmonic Prolongation Via Fifth Relation Cycles