Is Your Square Actually Square? Test It, Because Nothing Else Will

Plane one edge of a board dead straight. Hold the square's stock against that edge and strike a line across the face. Now FLIP the square over, stock still on the same edge, and strike a second line from the same point. If the square is true the two lines lie exactly on top of one another. If it is out, they diverge — and they diverge by DOUBLE the error, because flipping doubles it. That amplification is what makes a tiny error visible. This needs no master square and no measuring. It is the same idea as winding sticks: use the geometry to multiply an error you cannot otherwise see. Do it to every square you own, today.

A TRY SQUARE is fixed at ninety degrees with a thick stock, and it is the fastest and most reliable for striking square lines and testing an edge. A COMBINATION SQUARE has a sliding head, so it also sets a depth, marks a line parallel to an edge, finds a centre and gives forty-five degrees. That versatility costs rigidity: the head can shift, and it is the one most likely to be out. An ENGINEER'S SQUARE is a precision instrument, usually small, made to a stated tolerance. Keep one purely as a reference to check the others against, and do not use it as a working tool — a square dropped on a concrete floor is no longer a reference for anything.

A sliding bevel holds any angle you set it to. Set it from the work itself — off a dovetail template, off an existing piece, off a full-size drawing — and carry that angle to every part without ever knowing what it is in degrees. That is the marking-gauge principle again: the two parts agree because they came from one setting, not because the number was right. Lock it firmly and check it has not moved partway through a set of parts. A bevel that slips halfway through marking out eight dovetails produces four that fit and four that do not, and nothing about the work will tell you which is which.

A square is a reference, and a reference that has been knocked is a liability rather than a tool — because you will go on trusting it. Hang squares rather than dropping them in a drawer with chisels. Keep them dry and lightly oiled if they are steel. And re-run the reversal test after any fall, at the start of any job that must be accurate, and periodically anyway. A wooden square can be trued by planing its stock; a metal one that is out is usually beyond economical repair and should be marked clearly and retired. Do not keep a known-bad square in the rack, because one day you will pick it up.