“…Spanos [1], Gagatsis and Patronis [2], and Patronis and Spanos [3] have considered various situations, in which there was a lack of understanding of the meaning of algebraic expressions and of terms used to denote classes of geometrical figures. These situations are described and analysed, in our model, as a result of the difference between the codifications of the pupils and those of the teacher: because of this difference, some expressions have a large semantic width-a technical term introduced in our model-which means that these expressions are difficult to understand because they are interpreted in many ways.…”