Structure-building rule

Structure-building rule is a rule which does not change already specified information. This rule just fills in information which is unspecified.

Example
the feature [voice] is distinctive within the class of obstruents, but non-distinctive in the class of sonorants. The latter fact can be expressed by leaving [voice] unspecified for this class of sounds. In the course of a derivation [voice] can be filled in by a structure-building rule. A property of structure-building rules is that the input and output are non-distinct.

Links
Utrecht Lexicon of Linguistics