What kind of square are you talking about? A try square? If it lines up after flipping it on a straight edge, it is square enough for wood working. If it is a combo square, look for one with a cast iron base (or stainless or other hard metal). If tried against a known square, it can be adjusted with a thin file to get into square. (Filing down the slot where the scale goes though) Once square you're okay. Aluminum based combo squares are easier to align, but will wear out of tolerance more quickly. Stay away from squares where the graduations are stamped on the scale arm. The stamping causes bulges on the edge that can lead to problems, unless you flatten them (thinking of hand plane soles, ain't ya!). Etched markings make it easier to get it true if it isn't already so.
JMTCW
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