Coping crown molding is not thought of as a science, but more as an art. It involves a set of skills and techniques passed from master to apprentice, or less formally, from one guy on the job to another. After working with trim for a long time and talking to a great many carpenters, I have developed a good handful of crown-coping techniques, and I’d like to pass them along. Created out of need, the techniques are based partly on math, partly on common sense, and partly on learning from mistakes. The most-important concept to realize is that every crown profile is designed for a fixed wall and ceiling projection.
Once you have determined the projection, you need to reproduce the measurement accurately and consistently in your miter cuts and in the installation of the coped joint. This will always remove the complications brought on by irregular corners, bad framing, and lumpy tape jobs. Cut precisely, a coped joint is forgiving and fits every time.