The function of the phonological rules is to complete the process of specifying the sentence. The P rules, which are applied after all morphological rules, operate on representations which consist entirely of morphonemes (both fully as well as incompletely specified) and of boundaries. The P rules have the following results: a) They complete the specifications of all incompletely specified morphonemes; i.e., like the MS rules, they replace zeros with pluses or minuses. b) They specify all features which play no distinctive role in the language but are not randomly distributed. This, too, can be viewed as assigning pluses or minuses to zeros. c) They change the values assigned to certain features in particular contexts. As already noted, the P rules are of the form "rewrite x as y", without reference to the "derivational history" of the symbols.
- Halle (2004), Pag. 28

The phonological or P rules assign features on the basis of purely phonological criteria; they require reference only to features and phonological boundaries. They differ from MS rules in that they may reassign values to specified features in addition to specifying nonphonemic features.
- Halle (2004), Pag. 20-21