It follows from these definitions that a differentiable function "f"("x") has a critical point "x"0 with critical value "y"0, if and only if ("x"0, "y"0) is a critical point of its graph for the projection parallel to the "x"-axis, with the same critical value "y0." If "f" is not differentiable at x0 due to the tangent becoming parallel to the y-axis, then x0 is again a critical point of "f", but now (x0, y0) is a critical point of its graph for the projection parallel to "y"-axis.