Hi, Ron. The angles on the back of the molding determine how the molding will fit on the wall. Most styles of crown are molded to fit at about a 38 degree angle to the wall. Some people lay all this out and then cut inexpensive wood like a 2x4 as a filler piece to go inside the angle with a little clearance. They then install the cut 2x4 at the wall/ceiling interface and finally nail the crown to the filler piece. This makes installation easier.
Crown, or any molding basically, should be coped at inside corners. You will find a lot of info on the board about this. From the "*** ****'s Message Board" page hit ctrl-f, type the word crown in the small box in the window that appears and left click your mouse to find earlier references.