For some, it is a shocking revelation. But most Americans eventually come to accept the fact that a “two by four” board is actually 1.5 inches by 3.5 inches (38 x 89 mm). But is it really? How accurate are the dimensions of boards found at hardware stores and lumber yards? It’s the job of state and local inspectors to answer that question by making measurements periodically at retail locations, or earlier in the supply chain.