Door anatomy: jamb, stop, casing, reveal — and what "prehung" actually means

A door is the most repeated unit in a house, and the vocabulary around it gets slippery fast. The supplier's order form, the framer, and the finish crew often use the same word to mean slightly different things, and a loose word on the order can turn into an uneven reveal on the finished opening. This is the parts glossary the way our crew uses it on site: what each piece is, and where it decides how the door hangs.

Whether you're leveling a door package on a bid or spec'ing a run of openings for a build, this is the vocabulary that keeps it straight. For the full scope, see interior door installation.

The jamb: the frame the door lives in

The jamb is the finished frame lining the rough opening. The door hangs off it and closes into it, and it has three pieces and two working sides.

• Hinge jamb — the vertical leg the hinges screw to. It carries the whole weight of the door, which is why it's the piece we plumb and fasten first.
• Strike jamb — the opposite leg, where the latch catches. The strike plate mortises into it, and the door closes to an even gap against it.
• Head jamb — the horizontal piece across the top, tying the two legs together into a three-sided frame.
• Hinge side and strike side — naming a door by its hinge and strike sides, plus its swing, is how a door schedule stays honest. Get the handing right on the order and the unit shows up swinging the way the plan wants.

Flat jamb vs. split jamb

The jamb comes from the supplier one of two ways, and it changes how the casing lands.

• Flat jamb (one-piece) — a single solid jamb sized to one wall thickness. You case both faces separately after it's set. It's the standard on new work where the wall thickness is known.
• Split jamb — a two-piece jamb that telescopes to take up a range of wall thickness, with the casing already attached to each half. One half sets from one side of the wall and the other slides in from the opposite face. Split jambs earn their keep on remodels and on walls that framed out thicker than nominal.

The stop, the reveal, and the gaps

These are the small dimensions that decide whether a door reads like finish work.

• Stop — the applied strip run around the inside face of the jamb that the door closes against. On a prehung it's usually already tacked on, and setting it consistent keeps the door from rattling or binding.
• Reveal — the sliver of jamb edge left showing between the jamb and the casing, held to a consistent setback, commonly around 3/16". Even reveals are the tell of a clean case job, and we cover the idea in full in what are trim reveals.
• Margin — the gap around the door leaf itself: a consistent line down the hinge side, across the top, and down the strike side. That even margin is what your eye reads as hung right.

The reveal on the casing and the margin around the door leaf are two different gaps, and both have to stay consistent for a door to look set. A tight even reveal around a door with a tapering margin still reads crooked.

The door itself, and what "prehung" actually means

The slab is the door leaf on its own, with no jamb around it and nothing bored or mortised into it yet. A prehung unit is that slab already mounted in its jamb, hinges mortised and screwed, stop applied, ready to shim into a rough opening. That's the whole meaning of the word: hung before it reaches the job. Which one to order is its own decision, opening by opening, and we lay it out in prehung vs. slab doors.

• Slab — the door only, with no jamb and nothing bored or mortised into it yet. You supply or reuse the jamb, mortise the hinges, bore for the latch and strike, and hang it by hand. It's more work than a prehung and gives you more control over the final fit.
• Prehung unit — slab plus jamb plus factory-set hinges plus stop, assembled before it ships. You plumb it, shim it, fasten it, and case it.
• Leaf — the door panel itself, whether it's flat, panel, or glazed. A single door has one leaf; a double or French opening has two.

Mullion and multi-leaf openings

When an opening carries more than one door, or a door plus a fixed panel, a few more parts show up.

• Mullion — the vertical member between two leaves, or between a door and a sidelite. On a double door it's the meeting point the leaves close against; some mullions are fixed and some are removable.
• Astragal — the molding on the edge of one leaf of a pair that closes over the gap between them, sealing the joint where the two doors meet.
• Double and French doors — two leaves meeting in the middle, sharing one wide opening. The reveals and margins have to stay even across both leaves, which makes them fussier to hang than a single door.

The casing that frames it

The casing is the trim covering the gap between the jamb and the wall, running up both legs and across the head. The profile sets the whole feel of the opening.

• Legs — the two vertical casing pieces up the sides of the opening.
• Head casing — the horizontal piece across the top, either mitered to the legs or run square under a header block.
• Backband — a secondary molding run around the outside edge of flat casing to build it up and add depth.
• Reveal setback — the consistent step-off from the jamb edge the casing is held to, so a thin line of jamb shows all the way around.

Which profile you run is a style call, whether that's a mitered craftsman casing, a square-shouldered casing with plinth and header blocks, or a plain ranch. We break the options down in door and window casing styles.

How it goes in

The parts only matter because of how they set, and the order of operations on a door is what makes the difference. We set the hinge jamb first and get it dead plumb, front-to-back and side-to-side, because everything else references off it. Shim behind each hinge, fasten into the framing, and the door's weight is carried true before the strike side is even touched. From there we set the strike jamb to an even margin against the closed leaf, adjust the stop so the door lands without rattling, and check the reveal the whole way around before anything's nailed off for good.

Walls in the real world aren't plumb, and framing in East Tennessee is no exception. When the wall the casing dies into leans off the jamb, we scribe the casing to the wall rather than leave a gap, the same way we handle any out-of-square surface, walked through in how to scribe trim to crooked walls. On split-jamb units the casing is pre-attached, so the jamb itself has to be set true to the finished wall.

East-Tennessee humidity and the door package

Wood moves with the seasons, and East Tennessee runs humid enough that a door fit tight in a dry spell can bind when the air turns heavy in summer. We set margins with that swing in mind, keeping the gaps consistent but not so tight that a wet-season leaf drags on the jamb. On a house full of doors in Knoxville or out in the West-Knox neighborhoods, that seasonal movement is what brings a crew back for callbacks when the margins were set too tight in winter.

When one moulding and trim crew handles the whole opening, from the door and its casing to the base and crown that die into it, the reveals stay matched across the package. Every door on the job is measured against the plan and set to the same reveal, so the openings read the same from one room to the next.