What's inside
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A combination square is useful for more than marking a 90-degree line. Its ruler can measure timber thickness, and the head can act as a shallow depth gauge for checking rebates, grooves and shoulders. For those jobs, a square that locks firmly and reads clearly matters more than a long list of extra features.
What to check before buying
Most general-purpose combination squares have a 12-inch (300 mm) steel rule and a head with 90-degree and 45-degree faces. That size suits common stock, cabinet parts and site work. A 6-inch (150 mm) square is easier to carry and handy for smaller parts, but it cannot span wider boards as conveniently. Longer rules help with larger timber, though they take up more room and can be awkward to handle one-handed.
Look for a rule with readable graduations on both edges and a head that clamps without shifting the blade. A cast-metal head is common; weight is not automatically a sign of accuracy, but a loose or rough locking mechanism is a practical problem. If possible, slide the rule through its full travel before buying. It should move smoothly, then stay put when locked.
For repeated checking, verify the square rather than trusting its markings. Put the head against a straight edge and draw a line along the blade. Flip the square over against the same edge and draw a second line. If the lines diverge, the square is out of true or the reference edge is not straight. A square can also be tested against a known accurate square.
Checking thickness and depth
To check timber thickness, set the head against one face and bring the rule across to the opposite face. Keep the blade square to the stock; a tilted blade can make a board appear thicker than it is. Measure at several points, especially near the ends and across the width. A board that varies by 1 mm over its length may be acceptable for rough framing but troublesome for a close-fitting cabinet joint.
The square is also useful for comparing boards. Set the rule to a known thickness, lock it, then use the head and blade as a quick reference. This is not a substitute for a caliper when you need a fine measurement: rule graduations commonly resolve to 1/16 inch or 1 mm, and parallax or a blunt rule end can add error. For planed stock that must fit a dado, check the actual groove and workpiece together rather than relying on the nominal size.
For depth checks, use the end of the rule as a probe. Rest the head on the surface around a groove or rebate, slide the blade down until it touches the bottom, then read the depth against the head. Keep the head flat and avoid pushing hard enough to flex the blade. This works well for depths within the rule’s reach, but it is less convenient in narrow slots and can be hard to read when the head obstructs the scale. A dedicated depth gauge or a rule with a wider, more stable base is a better choice for frequent, precise measurements.
Which type suits the job?
| Type | Best use | Trade-off |
|---|---|---|
| Basic 12-inch combination square | General thickness checks, layout and occasional groove-depth checks | Good value, but the lock and scale may be less refined |
| Higher-quality combination square | Frequent marking and repeat measurements on furniture or joinery | Costs more; still needs checking for square and lock movement |
| Small 6-inch combination square | Small parts, tool-belt carry and quick bench checks | Limited reach on wide stock and deep cuts |
| Dedicated depth gauge | Repeated or more controlled depth measurements | Extra tool to buy and carry; less versatile for general layout |
Choosing a square
For a home workshop or occasional site use, a straightforward 12-inch combination square is usually enough. A budget square is fine for rough carpentry, setting a circular-saw fence or checking stock that does not need a close fit. The main risk is not a plain finish; it is a rule that slips under the clamp or a head that is not square.
If you make repeatable furniture parts or mark joinery often, consider a precision combination square. Better machining and a more dependable lock can save time when transferring dimensions, but do not assume the price guarantees accuracy. Test it on arrival, check that the blade does not rock in the head, and return it if the test lines fail to agree.
A 6-inch combination square makes sense as a compact secondary tool, not necessarily as the only square in the shop. It is quick for checking small blocks and setting short offsets, while a 12-inch rule is easier for measuring wider timber and reaching into deeper cuts.
Common mistakes and care
Do not treat the square’s depth reading as exact if the head is perched on sawdust, a raised edge or a rough surface. Brush the reference area clean and seat the head fully. In a narrow groove, the blade may touch a wall before it reaches the bottom; check that it is vertical and not leaning. For a critical cut, confirm the reading with a depth gauge or a marked scrap.
Keep the rule and head free of pitch and rust, and do not overtighten the lock. Excess force can damage the mechanism or make the blade difficult to adjust. Store the square where the rule will not be bent. A combination square earns its place because it combines measuring, marking and basic depth checking in one tool; buy the size and build quality that match the work, then verify it before relying on it for accurate joinery.