[...] let us examine the archetypical phonological rules that must be formulated transformationally: rules of metathesis. It has been observed both traditionally and in more recent studies (Ultan [Ultan, R., 1971, “A Typological View of Methatesis”, “Working Papers on Language Universals” 7, pp. 1-44])) that only a very limited set of possible metathesis rule types exists, depending on phonetic properties of the affected segments. One type is vowel-liquid metathesis, represented, for example, by the Maltese rule of Brame [Brame , M., 1972, “On the Abstractness of Phonology: Maltese ʕ”, in Brame, M., ed., “Contributions to Generative Morphology”, University of Texas Press, Austin, Texas]. This apparently reflects a more general type of metathesis between neighboring continuants of unequal sonority, as the Latvian vowel-glide metathesis of Halle and Zeps [Halle, M. and Zeps, V., 1966, “A Survey of Latvian Morphophonemics”, “Quarterly Progress Report of the Research Laboratory of Eletronics” 83, 105-113] shows. Another sort is stop-spirant metathesis, like the Akkadian rule [...]. An apparent distinct type, involving identic consonants separated by a vowel, is attested in the Classic Arabic rule [...]. It is fairly clear from these brief observations, as well as others by Ultan [Ultan, R., 1971, “A Typological View of Methatesis”, “Working Papers on Language Universals” 7, pp. 1-44], that there exists a quite limited set of possible metathesis rules, which we could characterize as a preliminary theory of natural metathesis.
- McCarthy (2004), a pag.264-265