If you buy one layout tool after a tape measure, make it a combination square. Not a speed square — though those are great too — but the old-fashioned kind with a sliding steel rule and a heavy cast head. For about the price of a pizza, you get a tool that marks dead-true 90-degree lines, checks 45-degree angles, sets blade and bit depth, tests whether a surface is flat, and even works as a makeshift depth gauge. Cabinetmakers have trusted it for over a century, and it hasn’t been improved on because there’s nothing left to improve.
This guide covers what the parts actually do, how to use each function without miscutting, how to check that your square is truly square, and what to look for when buying one.
The Three Parts (and What Each One Does)
A combination square is embarrassingly simple, which is the whole point. It has three parts:
The rule (blade). Usually 12 inches of hardened steel, etched with graduations on both sides. Most quality rules carry 8ths, 16ths, 32nds, and sometimes 64ths on different edges. The rule is the measuring half of the tool — and it’s also a decent straightedge for short layout work.
The head (stock). The cast chunk of metal that slides along the rule and locks in place with a knurled knob. One face of the head is ground perfectly flat and perpendicular to the rule (that’s your 90-degree reference); the other face is ground at 45 degrees. This single fact — one face for 90, the other for 45 — is the whole reason the tool exists.
The scribe (awl). Many heads have a small pointed scribe stored in a hole in the casting. It’s easy to ignore, but it’s the most underused part of the tool: a sharp point for marking exact lines without the slop of a pencil lead.
Marking a Perfect 90-Degree Line
This is the job you’ll do a hundred times a day, so get the rhythm right.
Slide the head along the rule to roughly the distance you want your mark from the board’s edge — say two inches. Tighten the lock knob snug, not gorilla-tight (over-tightening wears the locking pin). Hold the head’s flat 90-degree face firmly against the edge of the board so the rule lies across the face, then run your marking knife or pencil along the end of the rule. Done: a line that’s square to the edge to within a fraction of a degree.
Two refinements that separate clean work from frustration:
Mark from a jointed edge. The line can only be as square as the edge you register against. If that edge is wavy, your “perfect” 90 will wander with it. Always register off a straight, true edge or a freshly jointed face.
Use the marking knife for fine work. A pencil line has real width — half a millimeter or more of uncertainty on each side of the mark. A knife line is a single crisp cut that you can drop a chisel or saw tooth into exactly. For rough carpentry, a pencil is fine. For joinery, use the knife.
Marking 45-Degree Angles
Flip the head around and register the 45-degree face against the board’s edge. Now the rule crosses the face at exactly 45 degrees. Mark along it and you have the layout for every miter joint, every picture frame, every beveled apron.
One practical warning: on narrow stock, the 45-degree face can rock if you don’t keep it flat. Press the full width of the face against the edge with your fingertips, and take the mark in one smooth motion rather than pecking at it.
Setting Blade, Bit, and Depth — the Hidden Superpower
Most people think of the square as a marking tool, but working carpenters use it just as often as a depth-setting gauge:
Table saw blade height. Unplug the saw first — non-negotiable. Extend the rule out of the head, stand the rule on the tabletop next to the blade, and slide the head down until its underside just kisses the highest tooth. Lock it. Now hold that setting against the blade and crank the blade up until it touches the head’s underside. Your blade height now matches the setting exactly, with zero parallax error from squinting at a tape.
Router bit depth. Same idea, upside down. Stand the rule on the router table, slide the head down to the top of the bit, lock it, and compare against your target. It’s faster and more repeatable than a tape measure, and it reads in true increments rather than eyeball guesses.
Depth of a mortise or dado. Drop the rule into the cut, let the head rest on the surrounding surface, and read the depth right off the rule. This is more accurate than sticking a tape measure into the hole and far less awkward.
Marking repeated measurements. Need five shelf pins all at exactly 4 inches? Set the head at 4 inches once, lock it, and run it down the board marking each spot. Every mark is identical — no accumulating error from re-measuring each time.
Checking Flatness and Squareness of Your Work
Is this board flat? Lay the rule across the face and sight along it. Light leaking under the middle means a hollow; the rule rocking means a high spot. The steel rule is straight enough to reveal problems your eye can’t see on its own.
Is this assembly square? Hold the 90-degree face inside a box corner or against a drawer side. A sliver of daylight at one end tells you the joint is out — and tells you exactly which way it’s leaning, so you know which clamp to tighten.
Did my miter saw actually cut 45? Set the square’s 45 face against the cut end. If the saw drifted even half a degree, the face will rock instead of sitting flush. Check this when you first set up the saw, and re-check whenever you move it.
Is Your Square Actually Square? The Flip Test
Here’s the uncomfortable truth: not every square leaves the factory true, and drops onto concrete don’t help. Test yours before you trust it — the test takes twenty seconds.
Hold the 90-degree face against the edge of a known-straight board and draw a line along the rule. Now flip the square over (so the other side of the rule faces the same edge), register it against the same edge, and draw a second line right next to the first. If the two lines are perfectly parallel — no wedge of daylight between them — your square is true. If they diverge, the square is out, and the gap doubles the actual error, so even a hairline wedge means the tool needs attention.
If it’s out by a small amount, some woodworkers file the head’s face or lap it on sandpaper over a flat plate — but be honest with yourself about whether a $15 square is worth the surgery. Often the right answer is a better square, which brings us to buying.
What to Look for When Buying
The rule should be etched, not printed. Printed graduations wear off in a year of shop use. Etched marks are cut into the steel and last decades.
The head should be cast iron or hardened steel, not zinc. Heavy is good here. A head with real mass sits flat and steady against the work. Machined faces matter more than brand names — you can feel a well-ground face; it’s smooth and sits without rocking.
Graduations on all four edges. The best rules give you 8ths, 16ths, 32nds, and 64ths across the four edges. The 32nds are the sweet spot for fine woodworking — readable without a magnifier, precise enough for joinery.
Lock mechanism that actually locks. Cheap squares develop a wobbling head within months because the locking pin is soft brass or plastic. A steel locking pin in a quality head holds its setting through years of daily use.
Length. Twelve inches is the standard and the right choice for most shops. Six-inch models exist and are handy for small parts and tool setup, but a 12-inch rule handles layout on full-size boards. If you only buy one, buy the 12.
You don’t need to spend a fortune. The mid-range machinist-style squares from established tool brands have served professionals for generations. What you’re really buying is faces ground true and a lock that stays locked.
Keeping It True
A combination square is a precision instrument wearing a work boot, but it still deserves basic care:
Don’t drop it on concrete. The number-one killer of squares. If it takes a hard fall, run the flip test before trusting it again.
Wipe the rule after glue-ups. Dried glue on the rule makes the head drag and throws off sliding settings. A damp rag now saves scraping later.
Oil the rule occasionally. A drop of light machine oil on the rule keeps the head gliding and prevents rust in humid shops. Wipe off the excess so it doesn’t transfer to your work.
Store it flat or hung, not loose in a drawer. A drawer full of tools rattling against the head’s ground faces is a slow-motion way to ruin them. Hang it on a nail or give it a dedicated slot.
The Bottom Line
A combination square replaces four separate tools — a try square, a miter square, a depth gauge, and a small straightedge — in a package that fits in an apron pocket. Learn the flip test, trust the 90 and 45 faces, and use it as a depth setter on your saw and router. It is, without exaggeration, the tool most woodworkers would save from a burning shop after their hands.
