Coupling between the two degenerate modes of a square patch, caused by a square corner cutout, is examined. The effect of a corner cutout on the coupling between modes of two adjacent patches is also discussed. The problem is formulated using an integral equation in the spectral domain, and solved with the Method of Moments. Two types of basis functions were used; full domain functions, with the edge condition, and subdomain basis functions. Placement of basis functions around the corner cutout is discussed. Results are presented showing the coupling between modes on a single patch, coupling between the modes on adjacent patches, and the effects of gap coupling the patch to a 50 ohm feedline.