Window trim parts: stool, apron, sill, and jamb extensions explained

A window opening has more named parts than most takeoffs give it credit for, and half of them get called the wrong thing on site. The framer says one word, the supplier's schedule says another, and by the time the trim sub shows up the "sill" everyone's been talking about turns out to be three different pieces. When the words drift, the bid drifts with them and the finish crew ends up pricing the wrong scope. This is the glossary an install crew actually uses — each part of a cased window named the way it lands on the schedule, plus where a deep wall changes how it goes in.

If you're already past the vocabulary and deciding how to case the opening, our stool-and-apron vs. picture-frame guide is the method post; this one names the parts. For the full package on a build, see how we run moulding and trim as one scope.

The interior parts you actually trim

These are the pieces the trim crew installs and the painter finishes. Everything here lands on the interior face of the wall, on top of a window frame that's already set and flashed.

• Stool — the horizontal interior shelf at the bottom of the window, the piece most people point at and call the sill. It notches around the jamb and projects an inch or so into the room past the casing on each side.
• Horn — the ear at each end of the stool that runs past the side casing. The end grain gets a return so no raw end shows, and the horn is what gets scribed to the wall.
• Apron — the trim tucked under the stool, usually a length of the casing stock with returned ends. It caps the drywall below the opening and visually carries the stool.
• Side and head casing — the legs and the top, the same casing that wraps a door, landing down onto the stool.
• Reveal — the sliver of jamb left showing between the jamb face and the inside edge of the casing, usually 3/16" to 1/4". It's a deliberate setback, held consistent on all three casing pieces, and it's what gives the casing a crisp shadow line where it meets the jamb.

The reveal is the detail that has to match the doors. Casing that sits flush to the jamb on the windows and reveals a quarter inch on the doors reads as a mistake from across the room. For how the profiles carry that reveal, see door and window casing styles.

Stool vs. sill: two different parts

This is the one that causes the most confusion on a schedule. The stool is interior trim. The sill is a different part entirely — it's the sloped exterior shelf at the bottom of the window unit, pitched to shed water away from the building. On a modern manufactured window the sill is part of the unit that comes from the factory, and it's not something the interior trim crew builds or replaces. When a plan or a client says "sill" and means the shelf you set a coffee cup on, they mean the stool. Keeping those two words straight keeps the exterior weatherproofing scope from getting confused with the interior finish scope.

Jamb extensions: the part deep walls force

A window unit ships with a jamb depth built for a standard 2x4 or 2x6 wall. The trouble starts when the wall is deeper than the jamb — and in East Tennessee that's common. Older Knoxville houses with plaster over lath, brick-veneer walls, block, and any assembly with continuous exterior insulation all push the finished interior face past the back of the window frame. When that happens, the casing has nothing flush to land on, and the opening reads like a hole in a thick wall.

The fix is a jamb extension — a strip of stock, milled to the exact gap, that runs the window frame out flush with the finished drywall so the casing can wrap it. On our Knoxville moulding and trim work, deep-wall openings are the rule on remodels and on any masonry build, so extensions get worked into the takeoff up front instead of discovered on install day.

• Order them to depth when you can — many suppliers will mill extension jambs to a specified reveal-to-wall dimension. That's cleaner than ripping strips on site and holds a tighter tolerance.
• Measure the real depth, in several spots — walls aren't a single number. Frame-to-finish depth can vary an eighth or more across one opening, so the extension either gets ripped to the widest point and scribed, or set proud and planed flush.
• Keep the extension flush or a hair proud of the drywall — the casing needs a flat, continuous surface to bridge the joint. An extension that sits below the drywall face leaves a step the casing rocks on.
• Account for the grade — stain-grade extensions have to match the species and be handled as finish stock; paint-grade can be a stable MDF or primed jamb material.

Extension jambs are a measure-first part. The frame-to-finished-wall depth has to be confirmed at each opening before anything is ordered or ripped — a jamb milled to the wrong depth turns every casing joint on that window into a fight.

Drywall returns: the other way to finish a deep opening

Not every deep opening gets an extension jamb and casing. The other approach is a drywall return, where the drywall wraps into the opening and dies against the window frame with a corner bead, and there's no interior casing or stool at all. It reads clean and modern, and it's the default on a lot of commercial interiors and contemporary residential specs. It's worth naming on the schedule early because it's a different trade hand-off entirely — the returns are a drywall-and-bead scope, and the trim crew's involvement drops to whatever stool or apron the design keeps, if any. Deciding return-versus-casing per opening before the drywallers finish keeps a window from getting mudded shut and then needing to be reopened for trim.

Scribing the stool to the opening

Once the parts are named and the extensions are set, the stool is the fussy install. It doesn't just drop in level — it has to be notched to fit around the jamb, the horns scribed to sit tight against a wall that's rarely flat, and the whole thing held level even when the rough opening isn't. On plaster and masonry walls the back edge of the stool gets scribed to the wall's real contour rather than trusting a straightedge, the same way any finish part gets fit to an out-of-square surface — the method is in how to scribe trim to crooked walls. Get the stool level and tight, and everything above it works off a true datum. The casing and the apron both reference that one line, so the reveal stays even the whole way around.

Tennessee humidity is the other thing the stool has to survive. Wood moves with the season here, and a stool run in the wrong material or set with no room to move will telegraph a gap or a cup by the first winter. That's a material and detailing question we cover in trim that survives Tennessee humidity.

Speccing a window package that installs clean

For a builder or a cabinet shop handing off a trim package, the window schedule wants a few things called out so the install goes in one pass: the casing method per opening (cased or drywall return), the reveal to hold, the grade, and — the one that gets forgotten — a note on which openings need jamb extensions and to what depth. On our end we install pre-built material to that spec; we don't supply the window units or fill the nail holes, since the fill and sand are the painter's scope. Send the plans and the window schedule and we'll bid the extensions, casing, stool, and apron alongside the doors and base so one finish crew runs the whole opening. Getting the depths confirmed at bid time is what keeps extension jambs off the punch list.