Proceeding from usual ionic radii by means of exponential averaging, Mean Fictive Ionic Radii (MEFIR) are reached and with these values Effective Coordination Numbers (ECoN) are calculated. In 'classical' cases, for example for closest-packed structures, the simple counted known values are obtained. Anion-anion-contacts and in the cases of cationrich compounds that have larger cations, contacts between cations, are also considered. Such calculations, applied only to parts of crystal structures, permit the detection and testing of structural relationships.