In PHONOLOGY, a development of FEATURE theory in which speech is modelled in terms of a series of independently functioning ARTICULATORS (lips, tongue front, tongue body, tongue root, soft palate, larynx), represented by NODES on separate TIERS. Articulator features are also called ‘place’ features, because they are grouped under the place CONSTITUENT in the feature HIERARCHY. LABIAL, CORONAL and DORSAL nodes represent single-valued features. Articulator-bound features depend on a specific feature for their execution, further specifying the nature of a CONSTRICTION formed by an articulator (e.g. APICAL and LAMINAL articulations are distinguished under the coronal node through the use of the features [ANTERIOR] and [DISTRIBUTED]). Articulator-free features (or ‘stricture features’) are not restricted to a specific articulator; they identify the degree of stricture of a sound independent of the articulators involved (e.g. [+continuant] sounds represent a continuous airflow through the centre of the oral tract, regardless of the location of the major stricture). Among the claims made for this model are its ability to offer an integrated account of vowel and consonant articulation in terms of place of articulation and stricture: for example, in one model, features such as back, high, and low, as tongue-body features, are linked under the dorsal node, and rounding under the labial node.