In the early days of American architecture, such ornamentation didn't come cheap; carpenters hand-planed their moldings. But after the Civil War, manufacturers used old munitions factories to mass-produce house parts, enabling builders to outfit even modest rooms from a catalog of affordable profiles. This trend climaxed with the high style of late-19th-century Queen Anne and Neoclassical houses.
Originally designed to mask cracks where plaster wall meets ceiling, crown molding bridges this seam diagonally, most commonly by tilting 38 or 48 degrees away from the wall. (This measurement is called the "spring angle"). The profile's more complex relief usually goes on the bottom creating depth at the ceiling. Adding molding above or below crown forms a more elaborate compound profile, while cove molding displays a simpler silhouette.