What square and plumb mean
Square means two surfaces or lines meet at exactly 90 degrees. Plumb means a line is perfectly vertical, in line with gravity. Level is the horizontal counterpart. These three conditions are the basis for almost every construction and woodworking task: doors that close, drawers that slide, walls that carry load, and boxes that do not rack.
The tools for checking them are cheap, but the tools themselves can be wrong, so good habit starts with checking the checker.
Checking a square for accuracy
A square can be dropped, bent or poorly made. A quick test needs nothing more than a board with a straight edge and a pencil.
- Press the stock of the square against the straight edge of the board.
- Draw a line along the blade.
- Flip the square over so the stock touches the same edge from the other side and the blade points the other way.
- Draw a second line along the blade, next to the first.
If the two lines match, the square is accurate. If they diverge, the gap shows twice the actual error. A small gap on a short blade can matter more on a long board. If you find an error, set the square aside for rough work, or take it to be adjusted if it is designed for adjustment.
Checking whether a workpiece is square
Using a square
Place the stock against one edge and look at the blade against the adjacent edge. Look for light between them; a thin sliver of light that tapers means the corner is not 90 degrees. Check both faces and, for thick stock, both ends, since corners can be square on the face and not square through the thickness.
Measuring the diagonals
For frames, boxes, doors and panels, measure corner to corner in both directions. If the diagonals are equal, the rectangle is square, provided the opposite sides are also equal in length. If one diagonal is longer, the frame is skewed toward that diagonal. To correct it, push the long corners gently together or clamp across the long diagonal and re-measure. This method is also how builders check a room or a deck, and our article on squaring a room goes deeper into using it at that scale.
For small objects you can compare diagonals with a story stick, a strip of scrap marked with the two lengths, or with a tape measure hooked on one corner. Measure from the same point on each corner each time, because hooking an inside versus an outside edge changes the reading.
The 3-4-5 triangle
For layouts too large for a hand square, the 3-4-5 triangle uses the fact that a triangle with sides in the ratio 3:4:5 has a right angle opposite the longest side. Mark 3 units along one line and 4 units along the line that should be perpendicular, then adjust the second line until the distance between the two marks is 5 units. Any consistent unit works; for example, 3 feet, 4 feet and 5 feet, or multiples such as 6, 8 and 10 for better accuracy over a longer span. The longer the legs, the smaller the effect of a small marking error.
Checking for plumb
Levels
A spirit level has a vial with a bubble that centers between two lines when the tool is horizontal. Most levels also have vials set for vertical. Hold the level against the surface so that it sits flat and does not rock, wait for the bubble to settle, and check the bubble sits between the lines. To check the level itself, read it, then rotate it end for end on the same spot. If the reading changes, the level is out.
Short levels are less reliable on uneven surfaces, because they follow every bump. A longer level spans bumps and averages them. A laser level can project a vertical line across a wall, which is useful when you need a reference over several feet, though you should confirm it against a known true reference before relying on it.
Plumb bobs
A plumb bob is a weight hung from a string. Gravity pulls the string straight down, so the string shows the true vertical. Hold the string at the top, let the weight settle and stop swinging, then measure the distance from the string to the surface at the top and the bottom. If the distances are equal, the surface is plumb. A plumb bob can be shielded from wind with a bucket or by working indoors, and a damped weight, such as one suspended in a container of water, settles faster.
Keep in mind that old buildings often settle and no wall in them is perfectly plumb. In that case decide whether you need things to be plumb to gravity or aligned with what is already there, because trim and cabinets can look wrong if they are exactly vertical against a crooked wall.
Good habits when checking
- Check before you cut, not after. A small error in a layout multiplies by the length of the cut.
- Re-check after clamping or fastening, because pressure can pull a frame out of square.
- Compare against a second method when the result matters, such as diagonals plus a square.
- Keep reference tools clean and dry so debris does not hold the tool away from the surface.
- Remember the limits of tools: very thin sources of error, such as a speck of dust or a burr, can show up as false results.
What to do when something is out
Small errors can often be corrected by planing, shimming or adjusting a joint before glue or fasteners lock it. Larger errors usually mean the parts were cut inaccurately and need remaking. It is better to learn this with a dry fit, assembling the parts without glue, than after the assembly is fixed in place.
This is general information, not professional trade advice. When a structure carries load or affects safety, follow the applicable instructions or codes and consult a qualified professional.