Closed baseboard corners An important principle with mouldings is that the casing should always be thicker than the baseboard. When this is not the case, we recommend you close the baseboard corner so that it becomes thinner than the frame. You should also close a baseboard whenever it meets an obstacle. To close a baseboard, saw it on a 45° angle on the flat and then cut it on a 45° angle in the opposite direction. Then mark a 90° line starting from the shortest corner and cut along this line to obtain the end that will close the baseboard.