Treating the nonlinear restorative force as a function F (x − x 0) of the displacement of x from its natural position, we may replace F by its linear approximation F 1 = F′ (0) ⋅ (x − x 0) at zero displacement. The approximating function F 1 is linear, so it will describe simple harmonic motion. Further, this function F 1 is accurate when x − x 0 is small. For this reason, anharmonic motion can be approximated as harmonic motion as long as the oscillations are small.