Tier Alphabet ( s ) : Tendencies and Restrictions

The subregular class of Tier-based Strictly Local languages provides a good fit for most of the natural language phonology. Its core idea is to capture longdistance dependencies by projecting the set of elements involved in a phonological process on a special tier, in order to establish a local relation among those remote units. This paper investigates the nature of the sets of elements involved in a long-distance agreement, i.e. tendencies and restrictions applied to the tiers. I give an overview of the long-distance harmonies that involve spreading of more than one feature, and show that there are certain configurations that are avoided. Mostly, vowel harmonies require just a single tier; consonantal (sibilant) harmonies need either a single tier, or two tiers, alphabets of which are in the set-subset relation; separate vowel and consonantal harmonies are captured by the tier-based strictly local grammars, tier alphabets of which are disjoint. Interestingly, there are no attested patterns where the tier alphabets only partially overlap. This topic is relevant for both for phonological theory and computational approach: apart from revealing new typological generalizations, it brings naturalness conditions to the formal grammars applied to the language.