According to Lieber, "The permanent lexicon consists of a set of all those terminal elements which cannot be decomposed into smaller parts" [Lieber, R., 1981, “On the Organization of the Lexicon”, Indiana University Linguistic Club, p. 38]. A class of rules that Lieber call "morpholexical rules" operates on this set. Lieber defines a morpholexical rule informally as follows. "A morpholexical rule is a relation defined between pairs of lexical items which are listed in the permanent lexicon" [Lieber, R., 1981, “On the Organization of the Lexicon”, Indiana University Linguistic Club, p. 39]. Furthermore, the members of this pair are related to one another in a special way: they are what Lieber calls "stem variants or morpheme variants".
- Aronoff (2004), a pag.59