An extension of the crystal coordination formulae has been proposed where the connectivity of the coordination polyhedra is considered. Possible structures are generated by systematically changing the connectivity of selected coordination polyhedra and the structures decoded according to this extended crystal coordination formula system based on the so-called intermediate notation (IN). Unfortunately, unrealizable atom arrangements may be generated in isolated cases. A method for selecting realizable crystal coordination formulae is suggested. The method is based on the hypothesis that any connection sequence of the coordination polyhedra should be closed for realizable structures. Practical examples for the selection method of polygon-connections are shown together with illustrations.