This applet is designed to show how the graph of a function in two variables can be constructed from the cross sections, which are each graphs of one variable.

• The applet has a control window and 4 graphics windows. 
• The first graphical window is the domain window.  It contains shows a grid of points, along with the cross section lines in the x-y plane.  The coordinates of the key point are (x0,y0) and they can be controlled a number of ways in the control panel.  The single arrows move the value by a single step.  The double arrows move the value through an animation to the end of the range.  A value can also be typed in.
  
• There are windows for an x-slice and a y-slice, the two cross sections of the 3-D graph obtained by holding y and x, respectively, constant at the hot spot. 
  
• The fourth window gives a graph of the surface.  It is worthwhile to step through the range for (x0,y0) while the style of the graph window is open surface.  This lets you see the wire from constructed from cross sections.  There are  check boxes on the control panel that lets you look at either the surface, or the intersection of the surface running parallel with the x or y axis, or the wireframe grid of the surface.  This lets you see that the wireframe is constructed from the cross section curves.