A small wooden box can look straight on the bench and still refuse to accept its lid. One common culprit is a frame pulled slightly out of square during clamping. Before the glue begins to set, compare the two corner-to-corner measurements rather than trusting the outline by eye.
Start with a dry assembly on a flat bench. Check that opposite parts match in length, the joints close properly and the corners are square. Measure across the same face from one outside corner to its opposite, then repeat across the other diagonal. Use matching reference points both times; switching between inside and outside corners tells you nothing useful.
On a properly fitted rectangular frame, matching diagonals are a useful check for square. They are not proof by themselves: unequal sides, bowed parts or an uneven assembly can still cause trouble. Keep the square nearby and check that all four corners rest on the bench without rocking. A tape measurement will not reveal every kind of twist.
Rehearse the adjustment before applying glue. Bring the clamps up just enough to hold the joints, compare the diagonals and ease the pressure before gently nudging the frame towards square. Reposition a clamp if it is pulling at an angle. Do not try to overcome a badly fitting joint with extra force; take the dry assembly apart and correct the fit instead.
During the actual glue-up, work within the adhesive’s stated assembly time. Recheck both diagonals after the final gentle tightening, because a frame can move as pressure increases. Check the corners and flatness again, then leave the assembly undisturbed for the specified clamp time. If the glue has already begun setting before a correction, do not wrench the joints sideways and assume they will remain sound.
Practical takeaway: make the diagonal check part of the dry run, not a last-second rescue. Use consistent measuring points, confirm the corners with a square and tighten only enough to close well-fitting joints. It is easier to correct a slight skew at the bench than to fit a lid around it later.
Box MakingGlue-UpsMeasuring