The metathesis of two phonological segments is the exchange of their syntagmatic positions in a larger phonological unit.

In metathesis, two segments swap their places. In transposition, one segment moves into a different place. Thus, abcd → adcb is metathesis, while abcd → acdb is transposition. Cases of abcd → acbd are theoretically undecidable; but on empirical grounds, they appear to be cases of transposition.

Examples: Latin miraculo ‚miracle’ and periculo ‚danger’ should have yielded Spanish miraglo and periglo, but the actual forms are milagro and peligro.

‘Ask’ is percontari in Latin, which gives perguntar in Portuguese. The Spanish counterpart preguntar results by transposition.

Theoretically, a distinction between distant metathesis (concerns non-adjacent segments) and contact metathesis (concerns adjacent segments) might be made. However, what might be analyzed as contact metathesis is actually no metathesis, but segment transposition. Consequently, there is no such thing as contact metathesis.