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What a Reveal Line Is and Why It Matters
When you install door casings, the trim doesn’t sit flush with the inside edge of the jamb. You leave a small step back – the reveal – usually between 1/8″ (3mm) and 3/16″ (5mm). That reveal hides slight variations in the jamb, gives the casing a shadow line so it looks intentional, and gives you wiggle room if the jamb isn’t perfectly flat. If the reveal wanders from 1/8″ at the top to 1/4″ at the bottom, the eye catches it immediately, especially where the miters meet at the head casing.
A consistent reveal is not about being fussy. It’s the difference between trim that looks like it was installed and trim that looks like it was nailed on. A combination square is the simplest, most reliable way to mark it because it references directly off the jamb edge and locks at a fixed distance.
Choosing the Right Square for Trim Work
You don’t need a premium machinist-grade square for this job, but you do need one that actually locks. The classic 12-inch combination square with a cast iron head and a steel rule is the standard for a reason. The 6-inch version is too short to register steadily against a jamb, and a 16-inch rule is unwieldy when you’re working overhead.
Look for three things: a head that slides smoothly but locks solid without shifting, a rule with etched markings that won’t rub off, and a flat, true 90-degree face on the head. If the head rocks or the rule has play when locked, you’ll get a wandering line. A $15-20 square from a big-box store will often lock with a thumb screw that slips after a few marks. A $35-55 square with a knurled brass or steel locking nut holds much better and is worth it if you do casing regularly.
For one or two doors, the cheaper option is fine if you test it first. Lock it, then try to push the rule with thumb pressure. If it moves, tighten harder or replace it. Don’t fight a loose square all day.
| Tool | Best For Reveal Lines? | Pros | Cons |
|---|---|---|---|
| 12″ Combination Square | Yes – best all-around | Adjustable 1/8″ to 1/4″ reveal, locks, references off jamb edge, doubles as 45/90 square | Can slip if cheap lock nut; needs checking |
| Marking Gauge (wheel or pin) | Yes – very consistent | Excellent for repeated parallel lines, no pencil wobble | Single purpose, needs sharpening/setup, wheel can dent soft jambs |
| Speed Square / Rafter Square | No | Great for crosscuts, fast | No adjustable reveal, thick body can’t ride jamb edge accurately |
| Small Sliding T-Bevel | No | Good for copying angles | Does not lock at a parallel distance |
| Pre-set Reveal Blocks (3/16″ plywood scraps) | Okay in a pinch | Cheap, no adjustment needed | Easy to tilt, no pencil guide, inconsistent pressure = inconsistent line |
Setting the Square to Your Reveal
Most trim carpenters work with a 3/16″ (4.5mm) reveal on paint-grade jambs and 1/8″ (3mm) on stain-grade or very flat pre-hung units. Pick one and stick to it for the whole house. Don’t switch mid-door.
To set the square:
1. Extend the rule out of the head until the distance from the inside face of the head to the end of the rule equals your reveal. The easiest way is to set it against a known reference: lay the rule on a flat surface and measure with a tape, or butt the head against a scrap of 1/8″ hardboard or two stacked business cards (roughly 3/16″).
2. More accurately, use the rule markings themselves. If you want a 3/16″ reveal, slide the rule until the 3/16″ mark aligns exactly with the edge of the head, then lock it. Check it with calipers or a tape if you want to be sure – errors stack up quickly and a 1/32″ slop at this stage means a visible wobble later.
3. Lock the nut firmly. Then double-check by holding the head against the edge of a scrap board and seeing if the pencil line you make measures correctly with a tape. Do this every time you pick the square back up after a break. They get bumped in a tool belt or bucket.
Pro tip: dedicate one square to reveals. Don’t keep resetting it for other measurements. Put a piece of blue tape on it marked “3/16 REVEAL” so you and anyone helping you doesn’t touch it.
Marking the Jamb Step by Step
Work on a jamb that is already shimmed, nailed, and checked for plumb. If you mark reveals on a jamb that still moves, your lines will be useless.
Hold the head of the square tightly against the inside edge of the jamb with your non-dominant hand. Keep the head flat – not tipped – so the full face is in contact. Any rocking will narrow or widen the reveal by 1/32″ or more. Apply light, consistent pressure toward the jamb edge as you slide.
With a sharp pencil – a 0.5mm mechanical pencil or a well-sharpened #2 with a chisel point – place the tip at the exact end of the rule and drag both the square and pencil together up or down the jamb in one smooth motion. Don’t move the square and then trace. Move them as a unit. The pencil should ride against the end of the rule, not angled away from it.
On the side jambs, mark from just above the floor to the underside of the head jamb. On the head jamb, mark from left to right between your side marks. You should end up with three light, continuous pencil lines that form a U. Those lines are the back edge of your casing – the edge closest to the door opening.
If the jamb has a slight bow or the edge is rounded over, keep the head on the flat part of the jamb, not riding on the radius. On soft pine jambs, don’t press so hard you dent the edge with the head. On MDF jambs, watch for crumbling edges that can give a false reference – vacuum the dust off the edge first.
For long runs or a lot of doors, a mechanical carpenter pencil with a hard lead leaves a finer, more consistent line than a soft carpenter’s pencil, which dulls after two jambs and thickens your line by 1/32″.
Checking for Consistency Before You Nail
Before you cut any casing, verify the lines. Close your eyes and run your finger over the reveal; you can often feel a variation you can’t see. Better yet, measure the distance from the jamb edge to the pencil line in three places on each jamb leg with a tape or the square itself. All three should be identical within 1/64″.
If one section is off, erase and re-mark it rather than trying to “average it out” when you nail. The casing will follow the line you give it. If you try to cheat the casing to hide a wavy line, your miters at the top will open up.
At the top corners, where the side reveal lines meet the head reveal line, make sure they intersect cleanly at 90 degrees. That intersection is your miter reference. If the square was tipped at the corner, the intersection will be rounded or overextended, and your miter will look off even if the cut was perfect.
Common Failure Modes and How to Avoid Them
The most common error is letting the locking nut work loose. Vibration from moving around does it. After every two or three doors, recheck the setting. The second most common is pencil angle. If you lean the pencil away from the rule, you add up to 1/16″ to the reveal. Keep the pencil vertical and always on the same side of the rule.
Another issue is jambs that aren’t co-planar. If one side jamb is shimmed proud of the head jamb by 1/16″, your reveal line will be perfect relative to each piece but the casing plane will be twisted. Sight down the jambs before you mark. If the face isn’t flat, fix the framing, not the line.
Finally, don’t use the square to mark the reveal on the casing itself. The square references off the jamb edge. Mark the jamb, align the back of the casing to that line, then nail. If you pre-mark the casing, any bow in the jamb will be telegraphed into a gap between casing and wall.
Used correctly, a combination square gives you a fast, repeatable reveal without jigs or blocks. Set it once, keep it locked, ride the head flat, and let the rule guide your pencil. The result is a tight, even shadow line on every door in the house with no measuring at the time of install.