Although most teachers adapt curriculum, we know little about teachers' rationales for modifying materials, how these rationales align with actual modifications, nor whether any patterns exist in the modifications that teachers make. This is especially the case in history/social studies, where research on curriculum is scant and research on teacher adaptation of curriculum is virtually non-existent. This paper addresses that gap. We report the results of a large-scale survey on curriculum use with over 1900 history teachers. The online survey focused on how and why teachers use and adapt lesson materials from a free online history curriculum and prompted teachers to upload examples of curriculum materials they had modified. We found that individual differences among teachers correlated with particular types of modifications. Moreover, we found that teachers were motivated to modify materials to address their students' needs, and that their modifications rarely affected the core structure -- or theory of content -- of the lessons. We argue that such alignment between teachers and curricular materials represents an example of "curricular fit." We discuss what curricular design features may have contributed to the high level of curricular fit among users as well as the implications of this construct for curriculum implementation efforts across subject areas.