For this demonstration, choose a point (r₀,θ₀) in the domain using the white hotspot in the window labeled "Domain". The second window graphs the function f(r,θ) and the r- and θ-slice curves at the point f(r₀,θ₀). The tangent lines to each of the slice curves at f(r₀,θ₀) 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(r₀,θ₀).