Commonly, Bookends have "Straight and Perdicular Sides" that corresponds to the sides of a book
Some Bookends have a flat protrusion extending from its backside to slide under the books. This is so that the books won't tip the bookend over
B&H Doorstops have 2 or 3 Protrusions on their backs as seen on the near left
B & H Castings that have NO Protrusions on its back are BOOKENDS as seen on the Right.