Many do-it-yourselfers have discovered that the two by fours (or 2 x 4s) they bring home from the lumber supply store are not actually 2 inches (5.08 cm) thick or 4 inches (10.16 cm) wide. The actual dimensions of this lumber can vary somewhat, but a true measurement of 1.5 inches (3.81 cm) by 3.5 inches (8.89 cm) is not unusual. Still, the designation is not necessarily a misnomer.
The lumber industry is nothing if not consistent with its measuring system. When harvested trees are brought to commercial sawmills for processing, the first cuts can be rough. Many of the irregular outer planks are discarded immediately. The usable lumber is often cut into easily divisible multiples of two, such as 24-foot (7.3-m), 12-foot (3.6-m), and 6-foot (1.8-m) lengths.
Indeed, at one point in the milling process, two by fours actually do measure 2 inches by 4 inches. This is the measurement of the planks just before they are run through a machine called a planer. A planer uses sharp blades to shave off all of the imperfect edges left behind by the rough sawing process. Commercial lumber mills may have to plane off as much as 0.5 inch (1.27 cm) of length and width from two by fours to provide a quality product for carpenters, roofers, and homeowners.
Older boards recovered from 100-year-old homes and other buildings may actually be true two by fours, however. Carpenters routinely planed their own lumber onsite to create a better fit between individual boards. Sawmills did not always use planers to create perfect boards — framers and carpenters were accustomed to working with rough-sawn planks of variable thickness and appearance. Planks were generally cut to standard measurements such as eight by eights, two by eights, and one by twos. The measuring terms still remain a popular reference, even if the actual dimensions are not entirely accurate.