Overestimates or Underestimates{{CMD_137}} Remember that geometrically, the actual function value corresponds to the {{MATH_138}}-coordinate of a point on the graph of the function and the approximate value corresponds to the {{MATH_139}}-coordinate of a point on the graph of the tangent line. We have terminology to describe the sitation when the graph of the function is above or below the tangent line: Concavity.