Door Frame Anatomy and Terminology
Standard door frames attach to specially constructed openings in stud walls. Studs that run the height of the door opening, called "jack studs," support the beam that crosses the top of a door opening, called a "header." The top portion of the jamb, called the "head jamb," fastens to the door frame's header, and the side portions, called "legs," fasten to the jack studs. Jambs generally protrude beyond the frame to accommodate the thickness of finished wall materials, such as drywall. The protrusion allows the doorjambs to rest flush with a finished wall surface. Casing attaches to the jambs and door frame and spans the gap between the wall covering and the jambs.
Although trim manufacturers offer door casing in a wide variety of dimensions, most casing exceeds 2 inches in width. If a door is near the corner of a room, a 2-inch-wide casing might fit between the jamb and corner. However, if a doorjamb abuts the corner of a room, standard casing will not fit. Even if a standard casing barely fits between a doorjamb and the corner of a room, painting the wall surface between the casing and corner is difficult, particularly if you plan to paint the wall a color that is different from the casing's color.
Lumberyards and home improvement stores stock a wide variety of trim that you can adapt to a casing installation project. Alternative trim pieces include baseboard moldings and wood strips, called lattice. Although not as thick and decorative as regular casing, a narrow piece of an alternative trim can effectively cover the gap between jambs and adjacent wall surfaces.
A skilled carpenter can create custom casing from wood stock, or modify standard casing to fit a tight space. A standard table saw allows carpenters to reduce casing width by creating long, consistent cuts. However, if carpenters make modifications to standard casing, they must ensure that all pieces are similarly milled; differences in adjacent casing pieces are easily noticeable.