Binary III-V surface reconstructions can be understood in terms of a simple electron counting model (ECM) : A surface structure satisfies this model if all cation dangling bonds are empty and all anion dangling bonds are full, given the number of available electrons. This model may be extended to hydrogen-covered surfaces and is satisfied by the majority of the stable ternary surface structures identified here. In fact, the 2×1-AlMD-2H structure is the only exception to the ECM. It is stable in a very narrow range of preparation conditions that are both cation and hydrogen rich.