Results from a project conducted in Mexico are discussed, in which a group of 17 indigenous teachers analyzed the numeration systems of their first language. The main goal of the project is to develop resources that help teachers in supporting students' understanding of the systems. In the first phase of the project, the central organizing ideas of 14 numeration systems were specified. Each system belonged to a different Mesoamerican language. Three aspects of the systems were identified that would have to be accounted for in instructional design. They include using 20 as a multiplicative base. Examples are presented of the instructional resources that indigenous teachers could use to help their students understand the quantitative rationales of the systems.