A correlation between the direction of the tetrad (double-double) effect in the lanthanide and actinide series and coordination number (CN) is discussed on experimental and theoretical grounds. Some examples of determining the CN values of the f-block elements in solution complexes using the tetrad effect are reviewed. Except for the CN implication, the tetrad effect of the inner transition elements is presented in terms of effective crystal radius, covalent shortening, nephelauxetic and ligand field effects, spin-orbit coupling, hydration, etc.