A 2x4 is not actually 2 inches by 4 inches, but rather 1.5 inches by 3.5 inches. This is due to the drying and planing process that reduces the size of the board from its rough sawn dimensions. The term "2x4" is still used to refer to this size of lumber because it is easier to remember and use than the actual dimensions. It is important to note that this difference in size applies to other types of lumber as well, not just 2x4s.