While /t/ metathesizes with a contiguous coronal sibilant in forms such as /hi+t+ ʃadel/ [hiʃtadel] ‘he endeavored’, no metathesis occurs when the coronal segments are tautomorphemic, as in /hi+tʃiʃ/ [hitʃiʃ] *[hiʃtiʃ] ‘he exhausted’. The crucial difference between the two sequences may be rooted in the observation that the coronal stop in the latter constitutes part of a morpheme, while in the former it is an entire morpheme in and of itself; thus, all information regarding the binyan 5 morpheme is contained in the affix /t/. If this consonant is weakly audible or entirely masked it is reasonable to assume that the identity of the entire morpheme would be jeopardized (Hume 1997).

Perceptual Optimization (Hume 1997, 1998): The observation that /t/ only metathesizes with a contiguous coronal sibilant is attributed to the acoustic and auditory similarity between the two consonants. The contiguous consonants at issue are similar in terms of sonorancy, voicing and coronal place, differing only in manner of articulation. Prevocalic position is a particularly favorable position for the realization of stop consonants, given the presence of both a vowel transition and release burst in this context. In preconsonantal position, on the other hand, no audible release of the stop is present. Thus, shifting the coronal stop /t/ with vulnerable cues to prevocalic position, at the expense of sibilants with stronger internal cues, strengthens the syntagmatic contrast between the stop and contiguous sibilant. This, in turn, may be viewed as a means of preserving the identity of the binyan 5 morpheme.