I always figured it was due to milling to finished dimensions from raw lumber. You can buy unmilled lumber by the board-foot (basically 1”x12”x12”), and you get

I always figured it was due to milling to finished dimensions from raw lumber. You can buy unmilled lumber by the board-foot (basically 1”x12”x12”), and you get exact pay for dimensions, but when you’re buying finished, milled lumber the only thing that you can trust is that it’s been milled to a standard. In other words, they start with ~2”x~4” and mill to exactly 1.5”x3.5” Don’t ask me why that all changes at 1/2” dimensions.