A two-by-four is a piece of lumber that is approximately 2 inches tall and 4 inches wide, but the length can vary. The standard lengths of two-by-fours are 8 feet, 10 feet and 12 feet.
Despite the name, the real dimensions of two-by-fours are 1 1/2 inches tall and 3 1/2 inches wide. When the lumber is cut, it is 2 inches tall by 4 inches wide, but the drying process causes it to shrink. The standard lengths are determined by the standard height of the frame in a wall, which is the most common use for this type of lumber.