The following is an example of one of the first exercises in defining used in the experimental approach (see Human & Nel et al, 1989b:21). EXERCISE 1(a) Make a list of all the common properties of the figures below. Look at the angles, sides and diagonals and measure if necessary. What are these types of quadrilaterals called? How would you explain in words, without making a sketch, what these quadrilaterals are to someone not yet acquainted with them?