|Abstract:||Recurrent sound patterns in the synchronic phonological typology often find analogs in diachronic development, and thus any account of this typology must include a set of explanatory principles to account for their existence over others in both domains. While most accounts acknowledge the importance of substantive phonetic factors such as articulatory ease and perceptual robustness, there is disagreement in the literature concerning whether these biases are due directly to aspects of speech production/perception or whether they are encoded abstractly in the mental grammar. Here I argue on the basis of a particular diachronic phenomenon, long-distance liquid metathesis in South Italian Greek, Gascon, and Sardinian, that a grammatical bias encoding (i) a desire for the enhancement of prominent positions and (ii) knowledge of perceptual similarity is crucial in explaining the nature of the sound change and its actuation. An analysis incorporating these biases is compared to two alternative, non-teleological accounts of actuation that have been proposed to explain the phenomenon: innocent misperception and the phonologization of motor planning errors. It is found that neither of these approaches alone adequately account for all aspects of the data. I suggest that while misperception events and motor planning errors are likely sources of innovative variants that may give rise to the sound change, selection among these variants ultimately relies on grammatical analysis.