This article portrays action research conducted in relation to students' faulty use of the equality sign in a Grade Four mathematics classroom in Sweden. Substantial background on teacher-researcher collaboration that prepares ground for the action research is offered. Drawing on cultural-historical and activity theory perspectives, the conduct of activities mediated by diagrammatic objects ("lappar") as cultural artefacts and students' inscriptions in these is also provided. As a semiotic activity, the action research cycle allows students to reflect on signs, meanings and their interrelationships while working with knowledgeable others. A three-stage conclusion discusses relevant research in mathematics education, the nature of practical theory realised, and actionable knowledge thereby possible in terms of trustworthiness. (Contains 2 notes and 4 figures.)