From Tonicization to Modulation

A secondary dominant tonicizes a chord for a moment, then the music comes home. Modulation is different: the music leaves and does not come back. The dividing line is the cadence. A tonicization just resolves to a chord. A modulation resolves to a chord and then proves it with a cadence in the new key, a clear arrival on a new tonic with its own dominant support. Once that cadence lands, every listener hears that the old key is gone. Retroactively, the chord that started the move now reads as the first step of the modulation rather than a passing borrowed chord.

Common-chord modulation is the smoothest way to make this move. You find a chord that exists, spelled the same way with the same function, in both the old key and the new key. That chord is the pivot: it can be labeled with a roman numeral in the old key and a different roman numeral in the new key, and either reading is honest. The composer treats it as belonging to the old key going in, then treats everything after it as belonging to the new key, confirming the shift with a cadence. Going from C major to G major, the pivot is often the chord built on C's vi or ii, since G major shares most of its triads with C major. From F major to its dominant, C major, candidates include F major itself (I in F, IV in C), D minor (vi in F, ii in C), and A minor (iii in F, vi in C).

Closely related keys, those sharing nearly all their triads, give composers the most pivot options and the easiest modulations for an ear to follow. Distantly related keys share fewer or no plausible pivots, so those modulations tend to sound more abrupt or rely on other techniques instead.