The true dimensions of a piece of finished 2-inch-by-6-inch lumber are 1.5 inches by 5.5 inches. The true dimensions of an unfinished piece of 2-inch-by-6-inch lumber is that of the actual nominal 2 inches by 6 inches.
The difference in the measurements in the finished and the unfinished products occurs in the drying process and the planing of the piece of lumber. The nominal size of a piece of lumber is always larger than that of its actual finished size. The actual finished size of a 2-inch-by-6-inch piece of lumber using the metric system is that of 38 millimeters by 140 millimeters.