In this paper, we explore how diagram placement in mathematics textbooks influences (i) students' and teachers' perceptions of exposition quality and (ii) students' reading behaviour as indexed by their eye movements. Our findings contribute to research on the "spatial contiguity principle," which recommends that related educational pictures and text should be displayed close together. In our first study, we used a comparative judgement technique to ask three groups of textbook users to compare real-world textbook expositions. Participants tended to perceive expositions as higher in quality if diagrams were placed in the main text rather than the margins. In a second study, we used eye tracking to explore whether students read expositions differently when diagrams were placed differently. Participants attended less to diagrams in the margins than to those in the main text. The findings of both studies suggest that authors should attend to the spatial contiguity principle when designing mathematics textbooks.