This cross-cultural study examines how elementary mathematics teachers in Finland and the United States shifted their instructional systems during the period of Emergency Remote Teaching (ERT) in the spring of 2020. Using Ruthven's (2009) framework of Structural Features of Classroom Practice, researchers analysed 15 teachers’ descriptions of their mathematics teaching during ERT in contrast to their typical practice. Exploring how teachers in both contexts navigated challenges of ERT through a cross-cultural lens allowed researchers to identify similarities and differences in teaching practices and use of activity formats, synthesized in composite models of teachers’ instructional systems. The differences in the systems can be traced to underlying cultural assumptions about the nature of elementary mathematics education and the roles of teachers, students, and curriculum resources within each system.