A point of inflection of a plane curve is a point where the curve has a stationary tangent, at which the tangent is changing from rotating in one direction to rotating in the opposite direction, i.e., a point where the direction of curvature changes. At such a point the second derivative of the curve's function has a value of zero, its value changing from negative to positive, or vice versa, between the two adjacent points on each side. See also extremum.