Visualisation is an important part of the cognitive process in mathematics. Pedagogical-psychological theoretical frameworks concerning cognitive processes consider perception and visualisation as basic conditions for the creation of correct mental imagery. This study examined visualisation as a key factor in the development of geometric concepts for pupils in primary and lower secondary education. The study has empirical-applicational characteristics. The results of research on Slovakian students regarding geometric concepts about rectangles and squares are described. The research sample consisted of two groups of pupils in the 4th grade (primary level) and 9th grade (lower secondary level). The comparison of results between the two groups enabled the identification of several misconceptions made by pupils regarding rectangles and several suggestions are included for educational interventions based on static and dynamic visualisation models.