It has been argued that teachers need practical principled knowledge and that design research can help develop such knowledge. What has been underestimated, however, is how to make such know-how and know-why useful for teachers. To illustrate how principled knowledge can be “practicalized”, we draw on a design study in which we developed a professional development program for primary school teachers (N = 5)
who learned to design language-oriented mathematics lessons. The principled knowledge we used in the program stemmed from the literature on genre pedagogy, scaffolding, and hypothetical learning trajectories. We show how shifting to a simple template focusing on “domain text” rather than genre, and “reasoning steps” rather than genre features made the principled knowledge more practical for the teachers.