This Old House general contractor, Tom Silva, teaches host Kevin O’Connor everything he needs to know about combination squares, how they work, and what they’re used for.
What Size Are Combination Squares?
Combination squares come in various sizes: 6 inches, 12 inches, 18 inches, and 24 inches. They have a steel ruler that slides back and forth over the slide handle, which has two built-in angles: 45 degrees and 90 degrees.
Anatomy of a Combination Square
There are a few parts of a combination square. The first is the steel ruler, which helps measure items and sets the square’s different surfaces to a specific size. The second part is the slide handle, through which the ruler slides and clamps in place.
The thumb screw locks the slide in place, allowing the user to repeat measurements and marks. There is also a scribe in the handle, which can be removed and used to make fine, accurate marks. Finally, some combination squares have levels built into the handle.
Common Uses for Combination Squares
Combination squares have multiple uses. They can be used for framing, where they can square lumber, mark 45 degrees, and help with layout. They can also be used for metal work, making accurate marks for cutting or drilling. They can also be used for laying out trim around window and door jambs. They’re also helpful for measuring the depth of dados or grooves and then transferring those measurements to create corresponding rabbets or tongues.
Quality Matters
Buying high-quality combination squares matters. Light-duty models are susceptible to bending or shifting with repeated use, meaning they’ll no longer strike square lines for layout. The heavier the combination square, the better the quality, and it’s a good idea to buy the best combination square you can afford.
Resources
Tom mostly uses a 6” combo square or 12” combo square. He looks for brands that are sturdy and not flimsy. Forged steel and cast iron are the strongest materials used.
He also showed a magnetic lock combination square, which can allow for quicker adjustments than the classic twist knob on the head.
TOH Tested: Types of Combination Squares
Jack of All Trades
Look past its gangly appearance and meet the most versatile layout tool going: the combination square. It’ll mark lines, measure thickness, check for square, and much more. Click through to see the five that deserve a spot in our toolbox—from heirloom-quality to bargain-bin.
What to look for:
- A blade with easy-to-read markings.
- A pleasantly hefty square head, preferably cast iron or forged steel.
- A knurled adjustment knob to lock or loosen the blade.
- A scratch awl at least 1 inch long that fits snugly into the head.
- Nicely machined edges free of pitting or paint.
Johnson 440
Price: About $30; johnsonlevel.com
At roughly one-quarter the price of an heirloom-quality combo square, this cast-iron workhorse, with a nonreflective finish, is the very best of the home-center fare. We like the threaded awl and easy-to-read green leveling vial but wish the sides of the head were better machined. It could also use a more prominent adjustment knob.
Empire e255
Price: About $8; empirelevel.com
This 6-inch-long pocketable square is easier to use for checking small parts or setting router-bit heights. The bright blue vial is a snap to read. Twist the chunky knob to loosen the square lock bolt, which can’t spin freely, and easily adjust the blade. The chamfered edges on the head make it comfortable to hold.
Swanson Speedlite TC131
Price: About $7; swansontoolco.com
Consider this the equivalent of a beater chisel, except with some nice touches—like a knurled brass knob—you wouldn’t expect to find on a sub-$7 tool. The readable blade and large vial are nice touches. And even though the head is plastic, it comes with a threaded awl that rests proud of the head—easy to grab.
LaGesse LaSquare LAS-12S
Price: About $36; lagesseproducts.com
At 2 inches wide, the aluminum head of this square won’t tip when you’re marking pipes and tubing. It’s also great for trueing a table-saw blade or checking an inside corner for square. That said, this isn’t the choice for marking 1× or other thin stock, since you’d have to prop up the material to meet the blade.
Starrett C33H-12-4R
Price: About $100; starrett.com
Nice to see that the company that introduced the combo square, in 1877, still makes one of the best. The forged-and-hardened steel head is paired with a blade etched with gradations filled with paint. A satin finish renders them easy to read even under the glare of your shop light.
Techniques for Using a Combination Square
Follow these tips from the This Old House team to learn the best techniques for using a combination square.
Check It for True
With a tool prized for its accuracy, it’s worth checking every now and then to make sure yours is true.
Extend the blade as far as possible, grooved side up, through the 90-degree face of the head, and hook the square against the straight edge of a board. Scribe a line along the blade. Now flip the tool over, groove down, and line up the same edge of the blade with your mark. If they’re parallel, the tool is square. If not, it needs to be repaired or replaced.
Mark and Check Notches
Set the blade to the notch’s depth. Hug the square to the board and, holding a pencil against the end of the blade, drag both along the board to mark the base of the notch. Use the blade’s long edges to mark the width. To check the depth of your cuts, position the square as shown, bottom out the blade, tighten the knob, and read the marking.
Find the Centerline
- Draw a line across the board 90 degrees to both edges.
- Use the 45-degree face to draw a line from the point where the 90-degree line meets an edge of the board.
- Flip the square over to the board’s other edge and draw another diagonal, as shown, from the opposite end of the 90-degree line. Where the diagonals intersect is the centerline.
Cut Sheet Goods Accurately
Unplug your circular saw, remove its battery, and flip it upside down. Place the square’s 90-degree face against the narrow edge of the shoe and extend the square’s blade until it touches one tooth, as shown, then tighten the knob. Read the marking and clamp a straightedge at that distance from your cutline on the keeper side. Cut away.