Tversky's (Tversky, A., 1977. Features of similarity. Psychological Review 84, 327–352) contrast model of similarity is used to examine the locus of two contextual effects on product judgments. More familiar stimuli may be perceived as more similar as well as more dissimilar than less familiar stimuli. Priming subjects' product knowledge should, meanwhile, increase the similarity of brands but decrease the similarity of categories. We argue that both of these context effects should be more prominent among the moderately proximal stimuli in any given set. Two studies, one examining the similarity/dissimilarity effect and one examining priming effects, support this conclusion. The observed priming effects were also stronger for brands than for categories. Overall these context effects appear more localized than previously assumed.