What's inside
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Why check a small try square?
A small try square is useful for checking cabinet parts, marking short shoulders and setting out lines across narrow stock. But if its blade is not square to the stock, every mark can carry the error into a cut or assembly. A small deviation may be hard to see on a short line and more obvious when transferred across a wider panel.
You do not need laboratory equipment to check one. A straight, stable board, a sharp pencil and a reliable reference edge are enough for a practical test. The check tells you whether the square is suitable for ordinary layout; it does not certify a precision instrument.
What you need
Use a flat-faced board with a straight edge at least 150 mm (6 in) long. A planed hardwood offcut or a verified straight piece of plywood is suitable. Avoid warped scraps, rough-sawn edges and soft wood that dents easily. You will also need a sharp pencil or marking knife, a ruler, and good light.
Before testing, wipe dirt, dried glue and burrs from the blade and stock. Check that the blade is firmly seated and that the stock does not rock on the board. A loose blade, damaged edge or raised rivet can make a square appear inaccurate even when the blade itself is straight.
If you are buying a square mainly for marking and checking furniture parts, compare common options by size and construction. A small try square is compact for bench work; larger squares give a longer reference across wider stock but take more room in a tool roll.
The draw-and-flip test
1. Set the stock firmly against the board’s straight edge. The blade should lie flat on the board, with its edge pointing across the face.
2. Draw a fine line along the blade, starting near the stock and continuing as far as the blade allows. Keep the pencil upright and do not push the square sideways as you mark.
3. Lift the square, turn it over so the blade is on the opposite side of the original line, and place the stock against the same board edge. The blade should now meet the line at its far end as well as at the stock.
4. Look for a gap between the blade edge and the line. If the gap opens steadily from one end to the other, the blade is not square to the stock, or the reference edge moved or was not straight. Repeat the test with a fresh line before judging the tool.
The flip matters because it reverses the direction of the square’s error. A line made with a square that is slightly out will diverge from the blade when the tool is reversed. This is more dependable than comparing the square against a board corner that you simply assume is accurate.
Read the result
On a short small square, the error may be difficult to see without magnification. Use a sharp pencil and a thin line; a thick carpenter’s pencil line can hide a small gap. A marking knife gives a finer reference, but use light pressure so it does not follow the grain or cut a wandering groove.
As a rough practical guide, a visible gap of about 0.5 mm (0.02 in) across a 100 mm (4 in) blade is enough to reject the square for close-fitting joinery. For rough framing or general marking, a smaller error may not matter. Do not treat that figure as a universal tolerance: the required accuracy depends on the work, and your line-making technique limits what the test can reveal.
For a numerical check, measure the separation between the original line and the blade near the tip with a ruler or feeler gauge. If the lines differ by 1 mm over 100 mm, the angular error is about 0.57 degrees. That sounds small, but the error accumulates over distance: a 0.57-degree deviation corresponds to roughly 5 mm across 500 mm.
Common causes of a false result
A bent blade can pass a check near the stock and fail toward the tip. Sight along both edges and inspect the blade against a straightedge. A damaged or rounded blade edge can also make the pencil track inaccurately even if the blade is square to the stock.
Other common problems are a board edge that is not straight, a pencil that leans differently on the two passes, and debris trapped under the stock. Repeat the test with the square in a different position on the board. If the result changes, suspect the reference surface or your setup before blaming the square.
Do not try to correct a riveted square by hammering the stock or blade. You can loosen the joint, twist the blade or create a new error. If the tool is out enough to affect your work, replacement is usually more reliable than an improvised adjustment.
Choosing a square for the job
| Type | Best use | Trade-off |
|---|---|---|
| Small try square, about 75–150 mm (3–6 in) | Short layout lines, small parts and tool-bag use | Easy to carry, but a short blade makes small errors harder to spot |
| Larger try square, about 200–300 mm (8–12 in) | Checking wider boards and marking longer lines | More reach, but bulkier and still needs an accurate blade-to-stock joint |
| Combination square | Adjustable 90-degree and 45-degree marking, plus depth checks | Versatile, but the head can be less convenient for quick one-handed layout |
If a small square is awkward to read or too short for the pieces you work on, a longer try square may be the better buy. For occasional rough work, an inexpensive square is often adequate if it passes the flip test. For repeatable cabinet or joinery work, choose a well-made square with a rigid stock and straight, undamaged blade, then check it before relying on it.
Keep it accurate
Store the square where the blade will not be bent or struck by heavier tools. Keep it dry, wipe off resin and glue, and avoid using the blade as a scraper or pry bar. Recheck it after a fall, after noticeable damage, or whenever a layout line starts producing a stubborn gap at assembly.
A square that passes the test is useful only when the reference edge and marking method are sound too. For critical work, check the square against a second known-good reference and verify the finished joint or cut before repeating the setting across a batch.