Writing in ThisIsCarpentry, Bill Shaw (an engineer who developed the CopeMaster) provides a great explanation of why copes work better for crown, and detailed descriptions of what works, and what doesn’t, when coping.
There is a great deal of misunderstanding about how copes work: for many carpenters, pressured by the need to ‘get the job done,’ cope joints are mysterious puzzles they haven’t the time or the patience to solve. But if we understand what makes a coped joint work then every cope can fit perfectly on the first try.