When installing crown molding trim, there are 4 types of different joints that you will encounter. Each joint serves a particular purpose and occurs at certain spots when decorating the upper corners of a room. The 4 joint types are coped, butt, miter, and scarf.
Butt joints are uncut, 90-degree ends that fit flush against an adjacent wall or surface
Coped joints work in conjunction with butt joints. With this type of joint, one piece of molding is intricately cut to match the profile of the adjacent piece, thereby making a corner joint that fits like puzzle pieces
Miter joints require a 45-degree angle cut on adjacent corner pieces. The cuts are made in opposite directions but form a 90-degree angle on an outside edge
Scarf joints are used to splice lengths of trim along a single wall