A simple valence or haplotype crystal is defined as a valence crystal of binary composition (A_{x} B_{y}), with one coordination number, (\mathrm{CN}(A)), for all cations and one, (\mathrm{CN}(B)), for all anions. Valence crystals have electroneutral formulas with respect to formal atomic valences (\left(V_{A}, V_{B}\right)) and each cation is surrounded entirely by anions and vice versa. Known haplotype structures include a majority of textbook examples of binary inorganic structures. Two simple rules apply to haplotypes: (x \cdot V_{A}=y \cdot V_{B}) and (x \cdot \mathrm{CN}(A)=y \cdot \mathrm{CN}(B)). These two rules strongly limit the number of possible combinations of atomic valence, composition, and coordination. A clear connection is shown to exist in known haplotypes between the coordination number of the majority atom and the dimensionality of strong bonding, which in turn has a great influence on crystal properties. In haplotypes with three-dimensional bonding, an observed relationship between composition and interatomic distances is shown to be a consequence of the rather regular coordination figures present in this group of structures. O 1993 Academic Press, Inc.