Found:

redundancy rule

PHONOLOGY/MORPHOLOGY: rule which fills in predictable or redundant information. Redundancy rules have two important properties: (a) they do not create structure, and (b) they do not alter structure. EXAMPLE: the fact that sonorants in English are always voiced, as opposed to obstruents, can be captured by leaving the feature [voice] unspecified, and fill in [+voice] by a redundancy rule. The idea behind redundancy rules and underspecification is that redundant information can be left unspecified in the grammar (usually the lexicon), and that a grammar which contains less (idiosyncratic) information is more highly valued than a grammar which contains more (every thing else being the same).

LIT.	Archangeli, D. (1984) Chomsky, N. and M. Halle (1968) Halle, M. (1959) Kiparsky, P. (1982) Stanley (1967)