Metathesis is generally understood as sound change involving the switching in position of two segments, or sequences of segments. For example, the non-standard English word ax ‘ask’ is related to the standard form by metathesis. But there are also some arguable cases where metathesis has involved the switching of individual features of segments, rather than the segments themselves.
For example, consider the Tocharian (Toch.) words for ‘tongue’: käntu in Toch. A, kantwo in Toch. B. From these two words we can reconstruct Proto-Tocharian (PToch.) *kəntwó; note that, following the convention of Ringe 1996, *ə denotes a high central vowel, not a mid central one as it does in the IPA. Now, the Proto-Indo-European word for ‘tongue’ is reconstructed as *dn̥ǵʰwáh₂1. The development of *-n̥- into *-ən- and *-wáh₂ into *-wo in PToch. is regular. However, the regular development of *d- in PToch. would be *ts-, and the regular development of *-ǵʰ- in PToch. would be *-k-. In other words, the expected PToch. form is *tsənkwó, not *kəntwó.
How can we explain this outcome? The first thing one might notice about the two forms is that where the PIE form has a coronal stop, the PToch. form has a dorsal stop, and where the PIE form has a dorsal stop, the PToch. form has a coronal stop. One might therefore suggest that the PToch. form comes from a metathesized version of the PIE form, with the coronal stop *d and the dorsal stop *ǵʰ having changed places: *ǵʰn̥dwáh₂. If *kəntwó is the expected outcome of PIE *ǵʰn̥dwáh₂ in PToch., then this hypothesis explains the outcome in the sense that it makes its irregularity no longer surprising; changes of metathesis are well-known exceptions to the general rule that sound change is regular.
Unfortunately, there’s a problem with this hypothesis: the regular outcome of PIE *ǵʰn̥dwáh₂ in PToch. is *kənwó, not *kəntwó, because PIE *d is regularly elided in PToch. before consonants. In fact there are no circumstances under which PIE *d becomes PToch. *t; if, by some exceptional circumstance, *d failed to be elided in *ǵʰn̥dwáh₂, it would probably become *ts, rather than *t, resulting in PToch. *kəntswó.
The solution proposed by Ringe (1996: 45-6) is to suppose that what was metathesized was not the segments *d and *ǵʰ themselves, but rather their place of articulation features. So *d became [-coronal] and [+dorsal] (like *ǵʰ), while *ǵʰ became [+coronal] and [-dorsal] (like *d). But the laryngeal features of the two segments were unchanged: *d remained [-spread glottis], and *ǵʰ remained [+spread glottis]. Therefore, the outcomes of the metathesis were *ǵ and *dʰ, respectively. And *kəntwó is, indeed, the expected outcome in PToch. of PIE *ǵn̥dʰwáh₂, because PIE *dʰ becomes *t in PToch. (There’s the interesting question of why *d becomes an affricate *ts, but its aspirated counterpart *dʰ is unaffected—but let’s not get into that.)
I did a search of the literature using Google Scholar, but I couldn’t find any other explanations of the development of PIE *dn̥ǵʰwáh₂ into PToch. *kəntwó. And I can’t think of any myself. Still, the scenario posited above is perhaps too speculative to allow us to say that metathesis of features is definitely possible. It would be better to have an example of metathesis of features which is still taking place, or which occured recently enough that we can be very sure that a metathesis of features took place. Ringe & Eska (2014: 110-111) give a couple of other examples, but both are from the development of Proto-Indo-European, and therefore not much less speculative than the scenario above. (It might be of interest that one of their examples is Oscan fangva, a cognate of PToch. *kəntwó; PIE *dʰ- becomes Oscan f-, so what seems to have happened here is the same kind of metathesis as in PToch., but with the laryngeal features switching places, rather than the place of articulation features.) Ringe & Eska do also mention that one of their daughters, at the age of 2, pronounced the word grape as [breɪk], thus exhibiting the same kind of metathesis as hypothesized for pre-PToch., i.e. with the place of articulation features being switched with each other but with the laryngeal features remaining in place.
- ^ Normally I would cite other reflexes of the proto-form in IE, but the reflexes of *dn̥ǵʰwáh₂ exhibit an amazing variety of irregularities, so that to do so would probably break the flow of the text too much. It has been proposed that *dn̥ǵʰwáh₂ might have been susceptible to taboo deformation, although it’s hard to imagine why the word ‘tongue’, in particular, would have been tabooed; then again, the fact that only a single IE branch (Germanic) appears to preserve the regular reflex of the root does cry out for explanation. I’m not sure how secure the reconstruction of *dn̥ǵʰwáh₂ (given by Ringe & Eska) is, although I don’t recall seeing any alternative reconstructions. The main basis for this reconstruction seems to be Gothic tuggō (which has become an n-stem, cf. gen. sg. tuggōns, but is otherwised unchanged) and Latin lingva (which has the irregular d- to l- change observed in a few other Latin words). But Old Irish tengae seems to reflect *t- rather than *d- (this is without precedent in Celtic as far as I know, but I don’t know much about Celtic), Old Prussian insuwis seems to have lost the initial consonant entirely. And as for Sanskrit jihvā́, the second syllable of this word is the perfectly regular outcome of PIE *-wáh₂, but the first syllable is either completely unrelated to PIE *dn̥ǵʰ- or has undergone more than one irregular development.
Ringe, D. A. (1996). On the Chronology of Sound Changes in Tocharian: From Proto-Indo-European to Proto-Tocharian (Vol. 1). Eisenbrauns.
Ringe, D., & Eska, J. F. (2013). Historical linguistics: toward a twenty-first century reintegration. Cambridge University Press.