In earlier times, there were various shirt-like garments with two corners on top and two on the bottom. The custom was not to affix tzitzis to these garments. Rema explains that this was because they did not have two sets of corresponding corners on the front and back of the garment. The Achronim are not satisfied with this reason, and they suggest rounding off one of the corners to clearly exempt such garments from tzitzis. Garments like frock coats, which have a slit in the back, should have one of the corners rounded off. If, aside from the corners created by the slit, they also have four corners on the top and bottom (adding up to six corners in total), three of the corners should be rounded off.
[שו"ע י, יב, משנ"ב לו, וביה"ל ד"ה הואי]