Abstract
A group G of local weights is constructed for the square, honeycomb, and triangular lattices which counts for any closed path in the lattice 1/2π times the change in the argument of the tangent vector (mod 2) and the number of enclosed units of area (mod 2). These weights are used to evaluate the partition function of the two‐dimensional Ising model with nearest‐neighbor interaction and with a particular, imaginary external magnetic field. For the square lattice, the method gives a result announced by Lee and Yang.