This applet is designed to build intuition about line integrals.

• The applet has a control window and 4 graphics windows. 
• The first graphical window is the domain window.  It contains a movable hot spot that focuses on a point in the graph.
• 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.  On the control panel there are check boxes to add the appropriate tangent line to each of these graphs.
• The fourth window gives a graph of the surface.  If the tangent lines are made visible on the slices, they will also be visible here.  An additional checkbox makes the tangent plane visible.  It should be noted that the tangent plane, if it exists, is the plane that contains the two tangent lines for the slice curves.