Crown molding is usually installed so that it appears to run seamlessly around a room. It's easier to do that with foam and plaster trim because their joints are filled; they don't need to be perfect. With wood crown, filling isn't an option—cuts have to be precise so that joints are virtually invisible. That's a challenge in the real world, where corners are never exactly 90 degrees and wood fibers are constantly swelling and shrinking. Here are two ways to keep joints tight.
Coping: This technique, used only with wood crown on inside corners, involves cutting along the profile of one strip so that it fits over the face of the adjoining one. It takes skill and time to make this cut, but it's much better than an inside miter at hiding a joint, and it's forgiving of out-of-square corners. Go here to learn how to cut copes
Corner blocks: These factory-made pieces, placed at outside and inside corners (shown), eliminate the need for coping or miter cuts; the crown ends just butt up against the blocks' sides. A slight bevel in the back of the crown ensures tight joints in out-of-square corners. Corner blocks come in different styles and can be used with any material. They do simplify joinery, but because they project slightly beyond the profile, they can interrupt a crown's continuity. TOH Pro Advice:
"To minimize your view of the joints in wood crown, make sure the copes and bevel joints point away from the room's main entry, where you might otherwise see shadow lines between sections." —Tom Silva, TOH