The interface stoichiometry and the wrong bonds between like atoms are important subjects of consideration in the study of grain boundary properties of compound semiconductors. The polar and non-polar interfaces are defined from the interface stoichiometry. A general definition of the interface stoichiometry has been presented. The atomic and electronic structures of one type of coincidence tilt boundary in S₁C, the {211} σ=3 boundary, have been calculated on the base of the reconstructed models by using the self-consistent tight-binding method. These have been compared with the previous results of the {122} σ=9 boundary in SiC. It has been shown that the wrong bonds make the boundary energy fairly high by the repulsive electrostatic energies and generate localized states at the band edges. It has been also shown that the different bond lengths of the wrong bonds affect the local bond distortions to a great extent at the interfaces, which determines the relative stability between the present models. The stability of the wrong bonds in the interfaces of general heterovalent compound semiconductors has been also discussed.