("the complete quadrangle experiment") draw a line ` and label three points a, c, b on this line. (it works better if c lies somewhere between a and b but this is not necessary). draw two new lines passing through a and call them m1 and m2. draw a third new line through b which intersects both m1 and m2 and call it n. set m1 ∩ n = p and m2 ∩ n = q. draw ←→ cp and label ←→ cp ∩m2 = r. draw ←→ cq and label ←→ cq ∩m1 = s. (p, q, r, s are called a "complete quadrangle".) draw ←→rs and label ←→ rs ∩` = d. repeat this process several times, each time starting with the same positions for a, b and c on `, but using different lines m1, m2 and n. what do you observe? can you think of any reasons that justify your observations? we will study why what you found occurs later in the course.