Not wrong, and yet not the best use of language for conceptual understanding

Area and Perimeter

It is curious that so many students when asked about perimeter and area say that “perimeter is the outside” and “area is the inside”. Generally, students know to add the side lengths for perimeter and multiply the side lengths for rectangles, but they miss the importance of perimeter and area being different types of measurement. The simplified language does not give them the distinction between perimeter measurement and area measurement. Perimeter is a one dimensional measurement which can be measured with a ruler or tape measure or string! Area is a two dimensional measurement which describes the size of the surface in squares. In other words it tells us how many squares (and bits of squares) of a certain size will fit within a described or defined shape or boundary. Area is two dimensional and the 2 dimensions (except for circles) need to be “perpendicular” to each other (at 90°); after all it is ‘squares’ we are measuring. This is why you never use slopey sides to calculate any area, it is always a base length and then the height that is "perpendicular". This enables the number of squares that make up the surface to be calculated correctly. The appropriate and correct use of maths language when teaching and learning maths is so very important.