Changes in humidity cause wood to "work". High humidity increases its volume, while low humidity decreases it.
The metal tongue of a ECE precision carpenter's square is riveted to the wood side. To ensure a precise and constant angle, there can be no play in the joint. If the square is stored in a damp place, and the wood expands, the joint comes under enormous stress and the square can be forced out of true. One can sometimes see the results on angles that have been stored improperly. After riveting the two pieces together, the manufacturer ECE sanded the wood again to ensure a flat surface. If the wood has expanded, the rivet heads will be recessed a bit in the wood.
Our manufacturer, ECE, does everything it can to minimize these kinds of problems. The wood is carefully selected. It must be straight-grained and mature. Careful storage and drying are a must before the wood is worked into a tool. After the tool has been finished, a lacquer finish prevents, in the short term, too much humidity from being absorbed into the wood.
After purchase, it lies with the customer to ensure that the angle remains true by storing it in a dry place. An unheated or only occasionally heated cellar is almost always too humid for storage, and an unheated shed is never a good place. If one's woodshop is like that, it is far better to keep valuable measuring tools in the house.
One cannot rely on one's impressions of humidity. But one does not need a humidity gauge to know if there is too much. If a light surface rust forms on steel that is not stainless, then the air is too damp!
Another tip: It is a good idea is to test the angle right after purchasing it. The easiest way to do it is to find a surface with a straight edge. Stretch a piece of string along the edge and compare the string to the edge to make sure it is straight (one would not believe how many supposedly "straight" edges aren't even close). Place the wooden side against the straight edge with the metal tongue on the flat surface. Draw a line with a pencil along the metal edge. Now flip the square over so that the other side of the tongue rests on the surface. If the line exactly follows the edge of the tongue, then the square has a true right angle. It is a good idea to repeat the test to make sure, as changing the angle of the pencil as one draws the line, for instance, can make the test inaccurate.