Alternatively: Crown moulding
Decorative molding used at the top of a wall where it meets the ceiling.
Crown molding is usually bought with asymmetrical 52° -- 38° bevelled edges. It is usually made out of wood, though foam and plastics are available. The visible surface is often decorated with ropes, dentrils, or contours.
Installation is much easier if one uses miter blocks for corners.
The molding itself can be used for many other home projects, for instance; building decorative shelves, boxes, or picture frames.