As an expert in the field of music theory, I can provide a detailed explanation of the difference between modulation and tonicization. Modulation and tonicization are both techniques used in music to create a sense of movement and variety within a piece. However, they operate on different scales and serve different functions within the structure of a composition.
Modulation is the process of changing from one key to another within a piece of music. This is a larger-scale operation that involves a shift in the tonal center of the music. When a modulation occurs, the composer is essentially starting a new section of the piece in a different key. This can create a sense of surprise, contrast, or development, depending on how it is used. Modulations can be to closely related keys, such as the dominant or subdominant, or to more distantly related keys, which can create a greater sense of departure from the original key. The new key becomes the temporary tonic, and the music will typically establish this new key through the use of cadences and other harmonic devices before returning to the original key or modulating again.
Tonicization, on the other hand, is a more localized technique. It involves treating a chord that is not the local tonic as if it were the tonic for a brief period. This can create a momentary sense of being in a new key without actually changing the overall key of the piece. Tonicization is often used to add interest or to create a pivot point from which a modulation can occur. It can also be used to heighten tension or to resolve a musical idea in a satisfying way. Tonicization typically involves the temporary establishment of a new tonic chord through the use of cadences or other harmonic progressions, but it does not involve a sustained shift to a new key.
Here are some key differences between modulation and tonicization:

1. Scale of Operation: Modulation operates on a larger scale, changing the overall key of a section of music, while tonicization is more localized, affecting only a brief passage within the existing key.

2. Duration: A modulation typically lasts for a longer duration and may encompass an entire section or movement, whereas tonicization is usually brief and resolves back to the original key or moves to a different key through modulation.

3. Harmonic Function: In modulation, the new key becomes the temporary tonic and is established through harmonic progressions that lead to a cadence. In tonicization, the new "tonic" chord is treated as if it were the tonic, but it does not function as the main tonal center for an extended period.

4. Sense of Departure: Modulation often creates a stronger sense of departure from the original key, as it involves a more significant shift in the harmonic landscape. Tonicization, while it can create a momentary sense of being in a new key, does not create the same level of departure.

5. Resolution: Modulations are often resolved by returning to the original key or by modulating to another key, whereas tonicization is typically resolved by returning to the original key or by moving to a different harmonic area within the same key.
In summary, while both modulation and tonicization involve the use of harmony to create a sense of movement and variety, they do so in different ways and on different scales. Modulation is a larger-scale operation that changes the overall key of a section of music, while tonicization is a more localized technique that creates a temporary sense of being in a new key without actually changing the overall key.
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In general, the difference can be thought of as one of scale (a matter of quantity, not quality). Tonicization refers to a situation where a single chord (other than the local tonic) is treated as a "new" tonic. Modulation refers to an extended passage in a new key.read more >>