It is shown that a special nonplanar two-dimensional ising model with short range two-spin interaction transforms to the spin equivalent of the baxter model. This is an example where the critical exponents depend on the value of the coupling constants in contrast to the universality so far assumed to be valid for this model.
A generalisation of the star-triangle transformation is used to establish a correspondence between a nonplanar Ising lattice with two interaction constants and the standard triangular Ising model. The critical point of the nonplanar lattice is obtained exactly for various values of the ratio of the interaction constants. The singularity in the zero-field free energy of the nonplanar lattice is investigated. It is shown to be of the same form as that displayed by the corresponding standard Ising model.