When looking at the graph of a surface, it is useful to look at the
three curves obtained by setting each variable to zero in turn.
Thus if we want to look at the plane
x+y+z=4
we look at the lines
x+y = 4 in the x-y plane
x+z = 4 in the x-z plane
y+z = 4 in the y-z plane
This applet is designed to let you look at surfaces and the level
curves
they make in the planes generated by the axes.