Abstract
The paper describes partial results of a research conducted in primary schools with 10-11 year old children. The research focused on the issue of the relationship between perimeter and area.
The main goal is to study the presence of a misconception documented by a number of researches and named "same A, same B", that is, in our case, "same area, same perimeter". Thinking about the relationship between area and perimeter of 2D geometrical shapes and the need to articulate the thoughts gives pupils the opportunity to grasp these two concepts deeper.
The results show how pupils understand the geometrical concepts of area and perimeter, which are part of the topic 'Geometry and measurement', which is one of the key fields of school mathematics. Suggestions on possible improvements of didactical approach to this topic are presented.