Door and window casings are trim units used at doors, windows, archways and framed openings throughout the home. They can range from highly elaborate to simple rectangular profiles and are typically made from wood or vinyl. The outer edge of these casings is usually flat so that it can fit flush against the home's baseboards and other types of wall trim. Most door and window casing products are interchangeable, so if you find a style you like, you can use it to trim almost any type of opening.
Casings create a decorative finish at doors and windows, and it can also help tie different architectural elements together to develop a cohesive style within a room. This trim also serves a practical purpose by concealing unsightly gaps around door and window frames. It may even help mask small flaws in the drywall that can occur when retrofitting doors and windows.
Doors are often trimmed with thin columns known as pilasters, though more basic trim designs are also available. The top of a doorway can be trimmed with an elaborate unit of trim known as an entablature. Entablatures can be more than 1 foot (30.5 centimeters) high and typically feature elaborate carvings or elegant designs [source: Vandervort].
Windows may be trimmed around the entire frame or just on three sides. The bottom of a window frame is often equipped with a shelf, known as a stool, then finished off with a section of horizontal trim called an apron.