The coupling between two identical square dielectric waveguides with parallel axes has been studied for various con®gurations, in which one of the guides was arbitrarily rotated with respect to the other. In one case, the ®rst guide was kept ®xed while the second guide was rotated from 08 to 1808. It was found that the coupling coe cient was maximal when the inclination angle was 08 or 1808 and vanished when it was 908. In another case, one corner of the ®rst guide faced one corner of the second guide. It was found that the coupling coe cient was less than half the maximal value obtained for the previous con®guration. This study has a practical aspect; it proposes a possible way to select an appropriate coupling in order to satisfy a given engineering design.