In both cases, the number of constants (four) correspond to the number of data points where "f" is given. The interpolant is linear along lines parallel to either the "x" or the "y" direction, equivalently if "x" or "y" is set constant. Along any other straight line, the interpolant is quadratic. However, even if the interpolation is "not" linear in the position ("x" and "y"), it "is" linear in the amplitude, as it is apparent from the equations above: all the coefficients "aj", "j" = 1–4, are proportional to the value of the function "f".