Tonicization

In Western music that is tonal, the piece is heard by the listener as being in a certain key.

The pitches used in a V/ii in this example include the notes A, C# and E (creating an A major chord).

However, it is important to note that the opposite is not true—just because there is an accidental does not mean that it is definitely a case of tonicization.

Taking the example given above with the chord progression V/ii → ii, it is possible to extend this sequence backwards.

Though perceptions vary [3] as a general rule if a chord is treated as the tonic for longer than a phrase before returning to the previous key area, then the treatment is considered a modulation to a new key.

"A secondary dominant is like a miniature modulation; for just an instant, the harmony moves out of the diatonic chords of the key.