Aus PopularScience.com "Two-by-fours are not actually 2-by-4—here’s why"
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Through the drying process, the boards naturally shrink, as moisture leaves the beams. The real shrinkage, however, comes when the “rough-sawn material” is sent to a planer, which rubs the surface of the wood down into the smooth shapes you can purchase at a hardware store. Without the rough edges, what went in as a 2-by-4 planks of rough-sawn wood is now a tongue-tripping 1.5-by-3.5, having lost approximately ¼-inch on all sides to the planer and drying processes. “Once upon a time, 2-by-4s really were 2 inches by 4 inches,” Stephens says. But these days, the blocks are smaller, prettier, and hopefully a little more environmentally resilient by the time they're up for sale.