This study investigates teachers' understanding of underlying unit structures including: the relationship between lessons and curriculum design principles; and the connections among lessons in a unit. We investigated: (1) teachers' understanding of curriculum design principles; and (2) the role of teachers' experiences with curriculum units in their understanding of underlying unit structures. Using clinical interviews, we identified patterns in teachers' understanding. We found that teachers

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... not understand design principles and the relationships between these principles and lessons well, although more experienced teachers seem to know more than less experienced peers. Teachers also have difficulty identifying connections among lessons and therefore the overall unit structures. Both of these findings are problematic for curriculum developers seeking implementation that is "true" to the intentions of the materials. This study helps to inform future design of curriculum units, professional development, and tools that help teachers understand unit structures.