Nobody has improved on the combination square

The inductee

Step forward, the combination square. A steel blade with a groove down its length, a cast head that slides along that groove, and a knurled nut that locks the two together wherever you want them.

It does more jobs than anything else its size. It checks a corner for square. It marks a line a set distance from an edge, over and over, without a tape or a pencil gauge. It sets a depth. It finds 45 degrees on the head's second face. Most heads carry a small level vial and a scribe in the body, so it can tell you whether a post is plumb or whether a shelf bracket is lying to you.

Machinists have used it. So have cabinetmakers, carpenters and every grandfather whose garage you've stood in wondering why his shelves were straight. Starrett is the name most people associate with it, and its form has barely moved in more than a century. Nobody keeps a design that long out of nostalgia. It stays because it works.

If you've read our piece on measuring and marking so the cut is right the first time, this is the tool that piece keeps reaching for.

Why it has never been improved on

The whole design rests on one idea. Two surfaces, the flat face of the head and the straight edge of the blade, meet at exactly 90 degrees. Everything else is a way of moving that reference to where you need it.

Because the blade slides, you aren't limited to one size. A fixed square gives you one corner. A combination square gives you the corner, plus a repeatable distance from any edge, plus a depth gauge, plus a miter reference. All from one locked-up piece of steel.

And the physics is simple. A hardened steel blade doesn't flex, doesn't stretch and doesn't care about humidity. The head is machined flat, so it seats against a board and stays there. There's no battery to die, no screen to crack and no calibration menu. The only moving part is a nut.

The best proof is the check you can run on any square. Pick a board with a straight edge. Press the head against it and draw a line along the blade. Flip the square over, press it against the same edge, and draw a second line along the first. If the lines sit on top of each other, the square is true. If they split like a pair of tracks, it's off, and you've learned that in ten seconds with a pencil.

You can't run that test on a laser or a phone app. That's the point.

What the "improved" versions got wrong

Fairness first. Some of the newer tools are good at specific things.

A speed square is quicker for marking rafters and stair stringers, and framers love it. It's a different tool for a different job, though, and it can't set a depth or scribe a line parallel to an edge. A digital angle gauge beats a protractor head when you need a precise bevel setting on a saw. Neither of those tools is a rival. They're colleagues.

The gimmicks are where it goes wrong:

• Plastic and lightweight heads. They're easy to carry and easy to bend. A head that flexes at the nut gives you a different angle every time you press on it.
• Blades printed with a pretty scale. If the markings rub off in a month, the blade is just a ruler with a drawing on it. Look for etched or engraved graduations.
• Digital readouts on a rule. They add a battery and a failure point to a job that a thumbnail and a clear scale already do well. They also tend to hide the one thing a square should show you: whether the work is flat against it.
• Lasers and projected lines. Useful on a job site for a long layout. Useless for checking whether a tenon shoulder is square, which needs a physical edge to rest against.
• Loose, sloppy locking. A nut that creeps under hand pressure is a reason to mistrust every line you've drawn since you last tightened it.

The pattern is the same each time. Somebody adds a feature, and the feature costs you the one thing the square did well: a flat, true, stubborn reference.

Buying one that lasts

You're buying two things at once, a ruler and a reference face. Both need to be honest.

Look for:

• A hardened steel blade, with graduations that are etched or engraved. Read what the maker publishes about the finish and the scale. A satin finish cuts glare under a shop light, and a fine scale only helps if you can see it.
• A head that's cast iron or machined steel, not thin stamped metal. Read the product description for how the head is made and finished. A ground, flat reference face is what you're paying for.
• A locking nut that bites without a wrench. The blade should slide smoothly and then stop. It shouldn't wobble in the groove.
• A scribe and a level vial you can actually use. Nice extras, but not worth paying for if the head's accuracy is poor.
• Replacement parts. Check whether the maker sells blades and heads separately. A tool you can repair is a tool you can keep for decades.

On brands, a long-established maker with a published accuracy standard and spare parts is a safer bet than an unknown name with a flashy listing. Read the maker's own spec sheet for how true the square is guaranteed to be, and don't trust a reseller's summary.

Check the current price, and expect a real gap between the budget tier and a precision-ground one. This is a tool to spend toward the top of your range on. A cheap square that's a hair out is worse than having no square, because you'll trust it. If you're building a starter kit, our tool tote piece is the right place to decide where this sits in the order.

Read the actual warranty before you buy. Look for how long it lasts, what it covers, what voids it, and whether you pay shipping on a claim.

Keeping it for the next generation

A good combination square asks very little. Wipe the blade clean after a session. Dust and moisture are the enemies, because rust on the scale eats the numbers you rely on. A light coat of oil or paste wax on the blade, wiped almost dry, handles it.

Treat it like the measuring instrument it is. Don't use it as a scraper, a pry bar or a straightedge for a utility knife. Every nick in the blade edge is a lie waiting to happen. And don't toss it loose into a drawer with chisels and files.

Hang it, or lay it in a drawer slot of its own. A fall onto concrete can knock a head out of true, so if one hits the floor, run the flip test again before you trust it.

Run that test once a year anyway. It takes a minute and a pencil.

Then hand it down. A square with a worn head and a scale that's lost its shine is a square that's been trusted. Put it in your kid's hand, show them the flip test, and tell them to keep it better than they found it.