There are two selection panels, one in the upper-right corner and another in the lower-right corner. The first provides choices on the models and rendering methods and the other provides choices that control the creation of a Bezier spline.
The panel contains two groups of radio buttons and one reset button. The first group of radio buttons is for selecting the type of model to be displayed while the second picks the rendering method. The wireframe method is only applicable when the model is either one of the surfaces of revolution or the extrusion in the z direction.
The reset button allows you to set the angles of revolution back to their initial values, namely, zero. Note from the description of the upper-left display window, you can rotate the geometry about the x and y axes by dragging the mouse in that display window.
Recall that a spline is a sequence of one or more curves where each curve is created from control points. Initially, when you click the left mouse button in the lower-left display window, a new control point is added to the current curve as the LAST control point of the curve. However, there are times when we may want to select and move an existent control point. Or you may want to insert a control point between two others. The group of radio buttons titled Points provides the mechanism for making this known to the program.
The spline may be displayed with lines drawn between the control points. Note that only the first and last control point is in the curve. A line drawn from either of these to its neighboring control point in the sequence is tangent to the curve. When you click on the radio button named Tangents Only, you will see the two tangent lines for each curve. If you select Convex Hull by clicking on its radio button, you will see all the line segments between neighboring control points.
Last, there are three buttons in the panel. The first is for deleting the entire spline (note that there is no "undo" mechanism). The second is to remove the last curve in the spline, which is the current curve being created. The third button communicates that we want to start a new curve, which is appended at the end of the last curve in the spline.