We denote horizontal and vertical spins (arrows on edges) in the eight-vertex model μ μ , α α respectively, and define up and right as positive directions. The restriction on vertex states is that the product of four edges at a vertex is 1; this automatically holds for Ising 'edges'. Each σ σ configuration then corresponds to a unique μ μ , α α configuration, whereas each μ μ , α α configuration gives two choices of σ σ configurations.