USING A MITER JOINT
At an inside corner, you might think butting two 45°-mitered pieces together would work, but usually an unattractive gap results. The walls probably don’t meet at a perfect 90° angle, so the miters won’t be tight—and wood will swell and shrink with changes in temperature and humidity. Caulking the gap before repainting ruins the clean joint. Using a coped joint avoids the problem.