The present paper studies the phonological process of metathesis in Persian. Metathesis is usually considered an irregular, sporadic and marginal process in some languages, but regular and rule governed in some others. It seems that metathesis can be regular when serving a "specific structural purpose". This structural purpose may be of a phonological nature. The analysis of metathesis in Persian shows that it used to be regular in Middle Persian and resulted in phonologically well-formed words. But in Modern Persian data, analyzed here, metathesis was found to be not regular. There are of course instances of metathesis that have resulted in phonologically well-formed structures, but there are, at the same time, some other instances of metathesis which have resulted in phonologically ill-formed combinations. It must be mentioned that in the data analyzed here, metathesis leading to phonological well-formed structures showed greater frequency.
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