The concept of phonological relationships has been central in most, if not all, theories of phonology. The goal of this dissertation is to determine the contributions of two factors, distribution and alternation, in leading speakers to group sounds as members of the same category. Using previously established methods of testing speakers' perception and processing of sounds--similarity ratings, discrimination on a continuum, and semantic priming--I investigate the processing of coronal fricatives in three different languages: (i) English, in which the contrast between s and sh may signal differences in meaning (as in see vs. she), though the two sounds participate in limited morphological alternations as in press/pressure; (ii) Korean, in which s and sh are in complementary distribution and participate in regular and productive morphological alternations; and (iii) Mandarin, in which s and sh are in complementary distribution but do not participate in allomorphic alternations due to Mandarin's lack of affixation and its phonotactic restrictions. The relationship between s and sh in Mandarin, due to the conflicting evidence from distribution and alternation, has been a matter of controversy. The results from the similarity rating experiment showed that both the Mandarin and English speakers rated s vs. sh as more different than did Korean speakers, suggesting that the Mandarin speakers, who have access only to distributional evidence, are less likely to treat s/sh as members of a single category than the Korean speakers, who are exposed to evidence from both distribution and morphological alternation. Furthermore, the judgments from the speakers of all three languages varied in different vowel contexts, suggesting that the assignment of two sounds as members of the same or separate categories is not necessarily absolute. These findings suggest that multiple factors contribute to the formation of phoneme categories and that phonological relationships are gradient rather than categorical.