[en] Niobium and Tantalum are found to exhibit a range of coordination numbers (CN) from 10 to 17 in their alloys with group III B and IV B elements. The III B and IV B elements in these binary alloys have CN ranging from 6 to 14. The polyhedra around Nb, Ta, III b, and IV b elements have been characterized and a description of the polyhedra in terms of certain symbols is suggested. The structures of the binary alloys considered have been described in terms of polyhedra packing. Most of the structures examined up until now could be built up with one or two polyhedra, only in a few cases more than 2 polyhedra are required. (Author)