OCP-Place in Arabic provides a classic example of a soft phonotactic constraint. The strength of the effect depends on the similarity and proximity of the target consonants. This talk will present a quantitative model of this interaction, and explore the ramifications of the model for phonological theory. The model applies and extends work on modeling similarity in cognitive psychology, suggesting that phonological knowledge is embedded in more general cognitive functions. It eliminates underspecification while still providing for the differential importance of redundant and nonredundant information. Lastly, it has implications for architectures (such as Optimality Theory) by which constraints are combined to predict outcomes.