Gestures involving constrictions of these three structures involve completely distinct, non-overlapping sets of articulators. Such alternations are typically not restricted to individual lexical items, but are general patterns exhibited by all lexical items that share some critical phonological property. The target becomes more like the trigger. They occur more in fluent, casual speech styles. In general, targets and triggers of phonological rules are not single segments, but classes of segments that share some speech properties.
Rules capture generalizations that hold over classes of segments.
These classes, called natural classes, show up in many phonological rules. Other gestures of the target are not affected . Membership of segments in natural classes is formally represented by features . Each segment is represented by a set of features are arranged hierarchically. The features map onto the properties of the gestural structure of speech, but postulation of particular features and hierarchical relations among them comes primarily from cross-linguistic investigations of the natural classes that are observed in phonological rules, not the gestural structure, per se.