As with agreement in morphology, phonologically conditioned alternation, such as coarticulation and assimilation add redundancy on the phonological level. The redundancy of phonological rules may clarify some vagueness in spoken communication. According to psychologist Steven Pinker, "In the comprehension of speech, the redundancy conferred by phonological rules can compensate for some of the ambiguity of the sound wave. For example, a speaker may know that "thisrip" must be "this rip" and not "the srip" because in English the initial consonant cluster "sr" is illegal."