What's inside
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What you are checking
A timber edge can look straight and still be out of square to the face. That matters when you butt boards together, fit a rail into a joint, or use one edge as a reference for a saw cut. A try square and a combination square can both check the corner between a timber’s face and edge. The useful difference is not that one is inherently more accurate: it is how each is built, adjusted, and used.
A try square has a fixed blade set at 90 degrees to its stock. A combination square has a sliding, lockable blade and a stock with a 90-degree face; many also have a 45-degree face. Either can reveal a gap, but neither can tell you whether a long timber edge is straight along its length. For that, use a straightedge or sight along the edge separately.
When a try square is the better choice
A try square is quick to pick up and has no blade-setting mechanism to slide out of place. Its stock sits against the timber’s face while the blade crosses the edge. Hold the stock firmly against the face, then look for daylight between blade and edge. Check from both sides if the timber is wide enough: a gap that changes direction may point to a twisted or locally uneven edge rather than a simple out-of-square cut.
For ordinary site carpentry, a 150 mm blade is handy for short pieces and fitting work; a 250–300 mm blade reaches farther across broad boards. Longer blades make small errors easier to see over a wider span, but are more awkward in a tool belt and can flex if treated roughly. A fixed square is often the sensible low-cost choice if you mostly check 90-degree corners and mark short cuts. Browse carpenter try squares in a size that suits your usual stock.
The main drawback is limited reach and flexibility. You cannot slide the blade to a chosen width, and a short blade may miss a defect near the far side of a wide board. Also, a square that has been dropped can be out of true. Don’t assume a familiar tool is accurate just because it looks well made.
When a combination square earns its place
A combination square lets you move the blade along the stock and lock it at a useful length. That makes it convenient for checking edges on different timber sizes, marking repeated depths, and laying out short 45-degree lines. The trade-off is a locking mechanism: if the blade shifts while you work, your mark or check is no longer dependable. Keep the blade and slot free of packed sawdust, tighten the lock, and confirm the setting before relying on it.
For checking a timber edge, use the same method as with a try square: stock against the face, blade against the edge. Make contact without forcing the tool; pressure can hide a small gap or rock the stock on a rough face. A combination square with a longer blade can be useful for wider boards, but the blade’s reach does not make it a straightedge. Its 90-degree test checks the relationship at the point of contact, not the whole length of the board.
If you also mark repeated offsets or need a 45-degree reference, a combination square can replace several simple checks in the kit. If you only check corners, the extra adjustment may be unnecessary. See combination squares for carpentry and pay attention to blade length, stock fit, and whether the lock holds firmly.
Try square vs combination square
| Feature | Try square | Combination square |
|---|---|---|
| Best for | Fast 90-degree checks and marking | 90-degree checks plus adjustable layout work |
| Blade position | Fixed | Slides and locks |
| Common failure | Blade or stock bent out of square | Blade slips, or the blade and stock do not register accurately |
| Advantages | Simple, quick, usually less to adjust | Adjustable reach; often includes a 45-degree face |
| Trade-off | Limited reach and fewer functions | More parts to keep clean and correctly set |
Check the square before checking timber
A simple reversal test catches a square that is out. Put the stock against a straight, flat edge of a board and draw a fine line along the blade. Flip the square over, place the stock against the same reference, and bring the blade back to the line from the opposite side. If the blade diverges from the line, the tool is not reading square. A wide separation makes the error easier to see; keep the pencil line fine so its thickness does not confuse the result.
This test checks the square against itself and the reference edge, so a badly bowed board can mislead you. Use a known straight edge, or repeat the check in another position. On a combination square, repeat with the blade removed and reseated, then check again after tightening the lock. If the reading changes when you reposition the blade, the mechanism or fit is a problem.
For a practical job, a visible gap of around 1 mm across a 100 mm check is enough to question a joint or cut that needs to close neatly. That is not a universal tolerance: rough framing can accept more than cabinet work, and timber movement may matter more than a tiny square error. Treat the measurement as a warning to inspect, not an automatic reason to reject a board.
Choosing for the job
Choose a try square if your routine is checking and marking right angles, you want a simple tool, and the blade is long enough for your stock. Choose a combination square if you will actually use its adjustable blade for repeated offsets, different board widths, or 45-degree layout. A cheap combination square can be fine for rough carpentry if its blade locks without shifting and passes the reversal test. A more expensive tool is not automatically accurate; check it when new and after a hard drop.
Before buying, test the stock for burrs or dents that would stop it sitting flat, and slide the blade through its range if it is adjustable. In use, brush dirt from the timber face where the stock bears. A splinter or lump of dried glue under one corner can create a false gap that looks like an out-of-square edge. If the timber is rough-sawn, choose a clean reference face or plane a small, flat bearing area first. For wider boards and longer layout work, a long-blade combination square may offer useful reach, but don’t substitute it for a straightedge when checking lengthwise bow.
Whichever type you buy, keep it out of the bottom of a heavy tool bag where the blade can be bent. A square is a reference, not a pry bar or hammer. Store it clean, check it periodically, and let the timber—not the price tag or the tool’s name—decide whether the result is good enough for the fit.