Casing is the trim board that frames a door or window and covers the gap between the frame and the drywall. This one is the Colonial style: a traditional shape with soft curves and a stepped edge. Made from MDF, which stands for medium density fibreboard: a smooth engineered wood board with no grain and no knots, so it takes paint well.

• What it is: One piece of casing, 14 ft long, sold by the piece.
• Size: 5/8" thick by 2-3/4" wide.
• Profile (the shape the edges are cut to): A plain flat strip runs along the thinner edge, then the face curves out to a rounded, stepped detail along the thicker edge. Install it with the thinner edge against the door frame (or the window frame, if you are casing a window) and the thicker edge out on the wall.
• Finish: Primed white, so a base coat is already on and it is ready for your paint.
• How many pieces: Casing goes on both faces of the wall around a doorway. Each face takes two uprights of about 84 in and one header across the top, so a standard door uses four uprights and two headers. Three 14 ft pieces cover a standard door with some left over, about 1.5 pieces per face. Measure your opening before you order.
• About the photo: we picked an image that matches this trim as closely as we could, so it is a close look at the shape and finish. It is not a photo of the exact piece you receive, and the proportions can differ a little from the real thing, especially between sizes.