Many researchers have suggested that understanding novel noun phrases involves a process of conceptual combination in which people determine how two or more conceptsfittogether to form a new concept. One important way that people combine concepts is by property mapping, which involves asserting that a property of one concq)t is true of the other concept as in, "box that is striped" for "skunk box." A n experiment investigated the hypothesis that property mapping occurs by structural alignment in which mental representations are aligned or put into correspondence. T h e result of this process is primarily a set of matching elements (called commonalities) and a set of mismatching elements related to the commonalities (called alignable differences). The experiment compared property mapping definitions to the alignable differences listed by subjects in a comparison task which is known to involved structural alignment. Consistent with the hypothesis, there was a strong correspondence between property mapping definitions and alignable differences compared to another strategy in conceptual combination not thought to involve structural alignment (slot filling).