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Tonicization (temporary emphasis of a non-tonic pitch through applied chords) differs from modulation (establishing a new key center) in scope and voice-leading resolution patterns. Tonicization typically resolves back to the original key within a phrase, often through a secondary dominant resolving to its target. Modulation represents sustained shift in tonal center. Voice-leading analysis reveals the distinction: tonicizing passages maintain voice-leading continuity with the original key, while true modulations establish new harmonic frameworks.
From your study of secondary dominants, you know that V/V — a chord built on the second scale degree functioning as the dominant of the dominant — can briefly make the dominant feel like a temporary tonic. From your study of modulation techniques, you know that pieces can shift their key center over longer spans. The distinction between tonicization and modulation is not a binary switch but a question of duration and commitment, and voice-leading analysis is the tool that reveals which is happening.
Tonicization is a short-term borrowing. A secondary dominant (an applied chord like V/V or V/IV) introduces a chromatic note that acts as a temporary leading tone toward its target. That target briefly feels like a tonic, but the pull back to the original key is already built into the harmonic context. The voice-leading evidence: the chromatic note introduced by the applied chord is a momentary alteration that resolves and then disappears. If you analyze the passage with Roman numerals in the original key, the secondary dominant makes sense as a brief detour — it arrives, resolves to its target, and the progression continues. The original key has never really been abandoned; the chromatic note was a guest that came and left.
Modulation is a sustained relocation. Once a piece has modulated to a new key, the original key's chords begin to feel foreign rather than home. The voice-leading evidence: after the modulation, the tonal center of the original key no longer attracts resolutions. Cadences confirm the new key; melodic phrases close on the new tonic. Roman numeral analysis in the original key becomes awkward — you find yourself writing strings of chromatic chords, which signals that the original key frame no longer applies. At that point, you re-analyze in the new key. The pivot chord technique (a chord that belongs to both keys) is the common modulation mechanism; it is the moment of ambiguity where the music could belong to either key, and the direction it takes afterward reveals which key has won.
The practical way to distinguish them in real music is to ask: does the original tonic regain its gravitational pull quickly? In a tonicization, yes — within a measure or two, you are back in the original key and the chromatic alteration feels like a passing color. In a modulation, no — the music settles into the new key with new cadences, new melodic patterns, and new harmonic stability. The longer the music dwells in the new tonal area, and the more convincingly it closes in the new key, the more clearly it has modulated rather than tonicized. Beethoven and Schubert routinely make this boundary ambiguous as an expressive device — passages that seem to tonicize turn out to be the beginning of a full modulation, or vice versa. Hearing this ambiguity is one of the rewards of ear training at this level.
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