In this demo, the function X(t) = (t, abs(tan(t))) is graphed from -π/2 to π/2, along with the unit tangent vector, which is not defined at t = 0. The limit as t approaches 0 from the left makes an angle of π/2 with the limit of t as T approaches 0 from the right. This is an example of a critical point known as a "corner."