For this demonstration, choose a point (x₀,y₀) in the domain using the white hotspot in the window labeled "Domain: f(x,y)". The second window graphs the function f(x,y) and the x- and y-slice curves at the point f(x₀,y₀). The tangent lines to each of the slice curves at f(x₀,y₀) are also drawn, and it is clear that together they determine a plane. This plane, which is shown in yellow, is tangent to the graph at f(x₀,y₀).