A term in hierarchical models of phonology for a level of phonological representation. For example, in autosegmental phonology, parallel tiers of phonological segments are proposed, each tier consisting of a string of segments, and representing a sequence of articulatory gestures or acoustic transitions. In a tone language, for instance, tones are represented on one tier, which specifies features of tone and nothing else; other (non-tonal) features are represented on a separate tier. Features cannot appear on more than one tier, and thus tiers can be defined by the features found in them, as in the case of the phonemic tier, the skeletal tier and the X-tier. The number of tiers varies between models. In particle phonology, for example, there are five: syllabic, nucleus, timing, root and particle tiers. Terminology varies greatly among different models, as in the case of the tier handling information about articulation, which has been called a ‘featural’, ‘gestural’, ‘melodic’, ‘segmental’ and ‘articulatory’ tier.