Creating a hyperbola is similar to an Ellipse, except you are constructing two open curves that are symmetrical to each other, rather than an enclosed space. This deals with the difference of the focal points rather than the sum of the focal points like in an ellipse. It is like an inside out ellipse. Why this construction works is because it works with the definition: For two given points, the foci (point D and C) , a hyperbola is the locus of points (ex. point E) such that the difference between the distances to each focus is constant. Creating the two lines in step 4 and 5 allows the distance of each focus points to be equal distance from each there. Then when creating the locus from DE to CE, it creates the appropriate curves.