The use of non-harmonic tones is amongst the most expressive techniques at the composer's disposal with which to create tension within and between triads in a progression. Although usually notated by the composer, they can also be added in performance as an ornamental device (depending on the performance practice involved). Non-harmonic tones can be either diatonic or chromatic, and can be used in tandem with one another to provide even more tension. All non-harmonic tones are prepared and resolved by consonant tones (notes that are a part of the overall harmonic idea). In order to best understand their varying applications, we will group them into categories depending on their metrical/rhythmic articulation and the resultant emphasis.
1. Weak and Strong Articulations. It is important to understand that metrical/rhythmic articulation is dependent on the context of the music:
a) In this example, the quarters on beats two and four are considered weak in the context of non-harmonic tones.
b) However in the second example, the second eighth of each beat that would be considered weak while the first eighth of all four beats would be strong.
c) Here we have a mixed rhythmic texture. As a result, while in the sixteenth note group the third pitch (E) is strong, in the second quarter this same part of the beat it is weak being subdivided into eighths.
2. Weak Non-Harmonic Tones. There are five basic types within this category. For each, the approach and resolution can be inverted. For instance, the passing tone example is descending here, however this analysis is also applied to a corresponding ascending passage. Also, some theorists may use alternate nomenclature or languages when applied to some these non-harmonic tones (auxiliary tone instead of neighbor, the French echappée instead of escape, and changing tone instead of the Italian cambiata).
In all, weak non-harmonic tones tend to be less expressive, being articulated on unstressed beats. The passing and neighbor tones are both prepared and resolved by step. The escape is prepared by a step and is resolved by a leap in the opposite direction, while the cambiata reverses this paradigm. The anticipation is prepared by a step and is resolved by a repetition.
3. Strong Non-Harmonic Tones. There are three basic types within this category. Strong non-harmonic tones tend to be very expressive, being articulated on strong beats. Of these, the appoggiatura (Italian for leaning) is perhaps the most strident. Various theorists speak about this non-harmonic tone as needing to be prepared by a leap as a mandatory requirement, or that one without a change of tone from its preparation needs to be labeled as a prepared appoggiatura. The first statement is incorrect (although an appoggiatura can be prepared by a leap) while the second is a bit too pedantic for our purposes. The important aspect that needs to be learned by the student is that the appoggiatura is resolved down by step.
The suspension is nearly identical to the appoggiatura, except the non-harmonic tone is tied instead of being rearticulated. The retardation is similar to both of these strong non-harmonic tones, however the resolution ascends by a step.
4. Compound Non-Harmonic Tones. There are two basic types within this category, each based on concepts covered in previous categories. The common effect of these non-harmonic tones is that they temporarily realign the weak/strong relationship within the metrical or rhythmic disposition of the particular passage.
5. Other Non-Harmonic Tones. Although we will only speak about three types within this category, other irregular non-harmonic tones are possible. However, since their occurrence may be unique, they do not always have an appropriate theoretical label. The free-neighbor is called as such since it to appears without any preparation, while the accented passing and neighbor tones combine concepts of both the weak and strong non-harmonic tone categories.
For a more in-depth explanation of the effect of non-harmonic tones in composition, refer to the chapter on the Forces of Tonal Harmony.
