This collaborative action research, grounded in Reflection-and-Action in Communities of Practice, aimed to characterize the meanings negotiated by in service teachers regarding the estimation and measurement of rectilinear lengths in reflection processes oriented toward attention to diversity in mathematics class. The study was conducted within a teacher education diploma program with a subgroup of primary school teachers who planned, implemented, and reflected on activities involving Colombian banknotes. The results show an initial naive understanding of estimation, later reconfigured as a process prior to the use of measuring instruments, as well as an initial understanding of length and width linked to spatial position, later reformulated toward the recognition of the geometric and metric invariance of the rectangle. This article presents the negotiated meanings associated with teachers’ mathematical thinking and its relationship with forms of access, participation, and expression in the measurement of lengths.