What's inside
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What you are checking
A small assembly might be a drawer box, a frame, a cabinet door or a set of mitred corners. In each case, “square” can mean two different things: an individual corner is 90 degrees, or the whole assembly is square when measured corner to corner. A square helps with the first check and with marking; it does not, by itself, prove the second.
A try square has a fixed blade set at 90 degrees to its stock. A combination square has a graduated blade that slides through a stock, usually with a 90-degree face and a 45-degree face. Both can check a small joint, but their size and adjustability change how convenient they are.
The practical difference
For checking a small assembly, a try square is simple and quick: seat the stock against one face, then see whether the blade meets the adjoining face without a gap. A combination square does the same job at 90 degrees. Its blade can be extended or retracted, which helps reach across wider parts and makes it useful for marking measurements as well as checking corners.
That sliding blade is also a potential source of error. If the rule is loose in the stock, shifts while you work, or is locked at a slight angle, the reading is not trustworthy. A fixed-blade try square has fewer moving parts, though a damaged or poorly made one can still be out of square.
Quick comparison
| Tool | Best use on small assemblies | Main advantage | Main trade-off |
|---|---|---|---|
| Try square | Checking and marking short 90-degree corners | Fast, sturdy, no sliding blade to adjust | Limited reach and no built-in measurement scale |
| Combination square | Checking corners, setting offsets and marking dimensions | Adjustable blade adds reach and measuring use | More parts to keep clean, tight and correctly set |
| Either tool | Checking whether a box or frame is assembled square | Can check individual corners | Does not replace comparing diagonals |
Choosing a size
For drawer parts, small frames and general bench work, a 6-inch square is easy to control and fits inside tight corners. A 12-inch combination square is more versatile for cabinet parts and wider stock, but can feel awkward inside a shallow box. Blade length is not the same as useful reach: the stock has to sit flat on the reference face, and nearby sides or clamps may block it.
If you mostly check short joints and already own a reliable ruler, a 6-inch try square is a sensible, uncomplicated choice. For one tool that can check, measure and mark, look at a 12-inch combination square. A cheaper square is fine for rough layout if you verify it before relying on it; a premium label cannot compensate for a blade that has been knocked out of alignment.
Check the tool before checking the work
Do not assume a new square is accurate, or that a shop square stayed accurate after a drop. To test it, place the stock against a straight, flat board edge and draw a fine line along the blade. Flip the square over, register it against the same edge, and draw a second line beside the first. If the lines diverge, the tool is out of square. A visible gap of even 1 mm over a 100 mm run can matter on a small, close-fitting assembly.
With a combination square, also check that the blade slides without rocking and stays put when locked. Clean dust and pitch from the rule and stock; debris trapped at the shoulder can make the blade sit crooked. Do not use the end of the blade as a pry bar, and avoid storing the tool loose with heavier tools that can bend its rule.
Checking an individual corner
Seat the stock firmly against one workpiece, with no sawdust between the faces. Bring the blade to the adjoining piece and look for a gap along its length. Check near both the inside and outside edges if the joint allows it. A gap at one end can indicate a square problem, but it can also come from a bowed edge, a rounded corner or a piece that is not held flush.
For glue-up, check more than one corner and recheck after tightening clamps. Clamps can pull a box out of alignment, especially if pressure is uneven. Use a light touch: pushing the square hard against a flexible panel can move the work and create a false reading.
Checking the whole assembly
To check a rectangular frame or box, measure between opposite corners in both directions. The two diagonals should match. On a small drawer with diagonals near 300 mm, a difference of about 1 mm is a useful practical target for ordinary shop work; tighter joinery or a visible face frame may call for less. Measure from the same points at each corner, and keep the tape or rule straight rather than draped over an edge.
A square can help diagnose which corner is open, but matching diagonals are the more reliable check for the overall shape. If they differ, ease the assembly toward equal measurements before the glue sets. Recheck after clamps are snug, not just before them.
Which one should you buy?
Choose a try square if your work is mainly short 90-degree checks, you value a robust fixed setup, and you already have separate measuring tools. Choose a combination square if you regularly mark offsets, transfer dimensions or need one tool to handle several layout jobs. A small combination square is handy at the bench; a longer one reaches across broader parts but takes more room to use.
For occasional DIY assembly, a sound budget tool is usually adequate after a flip test. Spend more when you use the square daily, need a longer blade that locks securely, or depend on repeatable measurements across multiple parts. Whichever style you buy, keep it clean, verify it periodically, and use diagonal measurements when the question is whether the entire assembly is square.