How to Properly Use a Combination Square

Know the parts before you start

A combination square has two main parts: a flat blade (also called the rule) and a head that slides along a groove in the blade. A lock bolt or knurled nut on the head holds the blade in place. The head has two working faces. One sits at 90° to the blade and the other at 45°.

Many heads also include a small level vial and a scriber, which is a pointed pin that marks a fine line. Some products list these extras and some do not. For example, the PEC Tools 12 in set lists an integrated bubble level and a removable marking pin, and its head locks with a bolt for 90° and 45° positions.

Check that your square is accurate

A square that is slightly off will pass the error on to every line you draw. A quick check takes a minute and is worth doing on any new or dropped square.

Pick a board with one straight, smooth edge. Press the head tight against that edge and draw a thin line along the blade with a sharp pencil. Now flip the square over, keeping the head against the same edge and the blade in the same spot, and draw a second line. If the lines overlap into one, the square is true. If they form a narrow wedge, the head or blade is out of square, and you should not use it for precise work.

Also look at the blade edge and the head face for nicks, burrs or dirt. Even a small chip can tilt the head and throw a line off.

Set the blade and lock it

Loosen the lock bolt just enough for the head to slide. Move the blade until the graduation you want lines up with the edge of the head, then tighten the bolt. Check the setting once more after locking, because tightening can nudge the blade.

Read the scale straight on, not from an angle, so the marks do not seem to shift. Know which scale you are reading. The Accusize 12 in set, for example, lists 4R graduation (8ths, 16ths, 32nds and 64ths), while the SHALL kit lists a 16 in ruler with minimum graduations of 1/16 in, 1/32 in and 1 mm. A fine scale only helps if you read the right line on it.

Mark a square line across a board

This is the most common job. Follow these steps:

• Choose a straight edge on the board and wipe off dust.
• Hold the head flat against that edge with your thumb and fingers pressing it in.
• Slide the head to the spot where you want the line, or set the blade to a measurement and lock it.
• Keep the head pressed in while you draw a pencil line along the blade edge, using a light, steady stroke.
• For a finer line, use the scriber or a sharp pencil held at a slight angle toward the blade so the tip touches the edge of the blade.

Other jobs it handles well

A combination square does more than draw square lines. Once you have the basics, these uses come up often:

• Marking 45° lines for miters and trim by using the angled face of the head.
• Setting a parallel line by locking the blade at a set distance and running the head along an edge, with a pencil held against the end of the blade.
• Measuring depth, such as a groove, a rabbet or a hole, by resting the head on the surface and sliding the blade down until it touches the bottom.
• Checking that a cut end is square by seeing whether light shows between the head or blade and the workpiece.
• Setting the height of a saw blade or router bit by standing the square on the table and sliding the blade to the number you want.
• Checking level or plumb with the vial, if your head has one, in quick, rough jobs.

Choose the right blade length for the job

Blade length sets how far you can reach across a board and how deep a cut you can check. Our best combination square comparison puts blade length first for that reason. Match the blade to the widest work you do most often.

Material changes how the square feels in use. Hardened steel stays stiff and holds fine markings, while aluminum alloy is lighter to carry. Here is how a few listed products compare on published specifications.

Speed squares are a different tool

Some products on lists like ours are speed squares, which are single-piece triangles with no sliding blade. They are good for framing, rafters and quick 45° and 90° marks, but they cannot set a depth or transfer a measurement the way a sliding head can. If your work involves setting heights or measuring depths, you need a true combination square, or a kit that includes both. You can see which listed products are which in our combination square comparison.

Safety, care and common mistakes

The scriber point is sharp, so keep it capped or retracted when not in use and keep your fingers behind it when you draw. Wear eye protection when you work around cutting or grinding, and when you check the edge of freshly cut metal, because burrs can be sharp enough to cut skin.

When you set a saw blade or router bit height with the square, unplug the tool first. Never reach in to measure while it is plugged in, even if it is switched off. A level vial is fine for general layout, but never use a metal square near exposed live wiring.

To keep the square accurate, wipe the blade clean after use, store it where it will not get dropped or bent, and do not use it as a pry bar, a hammer or a scraper. Wipe a steel blade dry to prevent rust.

Frequently asked questions

Can I use a combination square on metal as well as wood?

Yes. The blade and head work on both, and a scriber leaves a clearer line on metal than a pencil does. Keep the head clean and watch for burrs on cut metal edges.

Why do my lines look slightly off even though the square passed the flip test?

The usual causes are a loose lock bolt, a head that is not fully seated on the edge, or a pencil held at a different angle on each line. Press the head in firmly and keep your pencil angle steady.

Do I need a level vial in the head?

Not for marking and measuring. It is a handy extra for quick level and plumb checks, but it is no substitute for a dedicated level on larger jobs.

Is a combination square the same as a try square?

No. A try square only gives you 90°. A combination square also has a sliding head, a 45° face and, on many models, a level and scriber.