The Metathesis reference article from the English Wikipedia on 24-Jul-2004
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Metathesis

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Metathesis is a sound change that alters the order of phonemes in a word.

The process of metathesis has altered the shape of many familiar words in the English language. Axe for ask goes back to Old English days, when ascian and axian were both in use. Some other frequently heard pronunciations in English that display metathesis are:

The process has shaped many English words historically. Bird in English was once bryd, run was once irnan, horse was hros, wasp is also recorded as wæps and hasp, hæps. The discrepancy between the spelling of iron and the usual pronunciation is the result of metathesis. Many other languages have words that show the phenomenon, and some use it as a regular part of their grammar (eg Fur.)

External links:

Ohio State University Dept. of Linguistics Metathesis Page: http://www.ling.ohio-state.edu/~ehume/metathesis/

Metathesis is used in chemistry as a synonym for double decomposition.