In this study, we focus on reading mathematical expressions and address the following research question: How do students observe and comprehend the characteristics of a mathematical object while reading mathematical expressions? To clarify the research question, we planned and conducted a design experiment. The lesson we designed dealt with a problem from “Jinkoki”, a Japanese mathematics textbook from the Edo period (Yoshida, 1977). Students’ thinking was analyzed in this study using a framework based on Tondorf et al. (2022) and Duval (2006). As a result, we identified a link in the representation, which is different from what Duval (2006) calls Treatment and Conversion.