Virtual equilibrium association constants for HA ^x models in cube representation. The virtual equilibrium association constants, Z_x, are indicated by the violet lines running from state 0 to the other states. The number of Z_x in a model defines directly the number of independent parameters for ligand binding. (A) HA² model. (B) HA³ model. (C) HA⁴ model. (D) HA⁵ model. The Z_x are indicated apart from the HA⁵ model where they were omitted for clarity. The dimensions of the Z_x are M⁻¹, M⁻², M⁻³, M⁻⁴, or M⁻⁵ for the occupation of 1, 2, 3, 4, and 5 binding sites, respectively.