In principle, a multipole expansion provides an exact description of the potential and generally converges under two conditions: (1) if the sources (e.g. charges) are localized close to the origin and the point at which the potential is observed is far from the origin; or (2) the reverse, i.e., if the sources are located far from the origin and the potential is observed close to the origin. In the first (more common) case, the coefficients of the series expansion are called "exterior multipole moments" or simply "multipole moments" whereas, in the second case, they are called "interior multipole moments".