In professional learning communities (PLCs) artefacts are important to mediate discussions of teaching development. Previous research has, however, shown that mediating artefacts for inquiry can act as obstacles in the joint work if the participants are unaccustomed to these artefacts. The mediating artefacts used to mediate the work in PLCs are typically pre-existing and seldom developed by the participants of the PLC. Contrary to this, in the study presented in this paper, teachers and researchers jointly developed a mediating artefact aimed to be useable in the planning, carrying out, and analysis of teaching for conceptual knowledge. The results indicate three characteristics of the process of developing this mediating artefact: a close connection to the practice, a precise mathematical content focus, and interaction between theoretical input and experience of teaching for conceptual knowledge.