 As an example, the electric potential (in spherical coordinates) due to a point charge located on the -axis at (see diagram right) varies as

If the radius of the observation point is greater than , the potential may be expanded in the Legendre polynomials

where we have defined and .This expansion is used to develop the normal multipole expansion.Conversely, if the radius of the observation point is smaller than , the potential may still be expanded in the Legendre polynomials as above, but with and exchanged.