Diffraction theory has been used to analyse a symmetrically truncated right-angle corner in a previous paper by Lewin and Crain. An inspection of the diffraction fields radiated by the conducting wedges which define this configuration has given insight into the symmetries which should be exhibited by this waveguide junction. It is emphasised here that, in a closed waveguide structure, all interaction terms must be taken into account to converge to a reliable solution. This has not been the case in solving open structures such as conducting strips in free space or open-ended waveguide horns. It is also seen that special care must be taken to account for fields in penumbra regions. This analysis reveals a symmetry of higher-mode generation by the corner when the height is an integral number of half wavelengths which could not otherwise be seen.