We address the question of how previously acquired linguistic knowledge facilitates perception and learning of a new language. We report results from two experiments showing evidence that participants better discriminate a segmental duration contrast in a novel language if they had some previous exposure to a language that uses duration contrastively. Crucially, the perceptual advantage occurs even when the novel language employs the contrast in entirely different conditions: in novel segmental contexts and for novel segments, including a change from application to vowels to application to consonants. We take these results to suggest that language learners use their knowledge of previously learned languages to make inferences about the ways in which languages are likely to vary, which in turn increases their perceptual sensitivity when languages do in fact vary in the predicted ways.