Speaking about harmonic domains, usually we mean a sequence of syllables in which harmonic vowels must belong to the same harmonic class. However, in the languages discussed in §4.1, the actual domain of harmony can be shorter than the potential domain, a sequence of syllables in which harmonic vowels may belong to the same harmonic class. In these languages, one of the harmonic classes is preferred (defalult). the members of this class can follow the members of the other class as well (but not vice versa). That is, there is a kind of vacillation, every time when a harmonic feature can spread from one syllable to another, there is a chance that it will not. Moreover, the more distant the syllable is from the initial one, the less the chance that a member of the dispreferred class will follow the previous syllable containing a member of the dispreferred class (and after some number of syllables, the vowels belonging to the dispreferred class are completely prohibited). We will call domains with harmony fading syllable by syllable harmonicity slopes.
The phenomenon is described by Wiik (1988: 189–195) who uses the metaphor of an electric torch with a discharging battery for such kind of decaying harmony. The vowel of the initial-syllable is the torch, the light of which diminishes (reaches a shorter domain), the light is weaker in some spots (centralized allophones instead of back and front ones) and the torch works only occasionally (vacillation).
In Finnic dialects with harmonicity slopes and in Udmurt, the preferred harmonic class is the back one. However, in Southern Khanty, the preferred class is the front one.