An interior door is installed in a

• A. Jamb
• B. Casing
• C. Lintel
• D. Sill

Answer: (A)

When you install an interior door, it sits in a door frame known as the jamb. The jamb is the vertical framing that forms the opening and provides a solid surface for mounting the hinges and the strike plate, helping keep the door square and aligned. The top horizontal piece that sits above the opening is a header (often called a lintel in structural terms) that supports the wall above, but the door itself is hung on the jamb, not on the header. Casing is decorative trim installed after the door and frame are in place to cover gaps between the frame and wall. The sill is the bottom piece, mainly associated with exterior doors to form a threshold and keep water out; interior doors typically don’t sit on a sill in the same way. So the door is installed in the jamb.