One common size of lumber is the 2-by-4, which is actually 1 1/2 inches thick and 3 1/2 inches wide. Another common size is the 4-by-4, which is 3 1/2 inches thick and 3 1/2 inches wide.
Lumber sellers usually describe board sizes in industry terms called nominal sizes, which do not reflect the lumber's true dimensions. Nominally, most lumber sizes range from 1-by-2 to 8-by-8. In actual dimensions, they range from 3/4 inch thick and 1 1/2 inches wide to 7 1/2 inches thick and 7 1/2 inches wide. When manufacturers make lumber into hardwood, which is usually installed in floors, they measure it in cubic inches of thickness. The sizes of hardwood boards range from 1/2 cubic inch to 3 3/4 cubic inches thick.
Lumber is simply timber produced from cut-down trees. Lumber yards trim the timber and separate the pieces by their characteristics to fit common uses. The two main forms of lumber are rough and finished lumber. Rough lumber refers to an unaltered, chopped-down tree, while finished lumber is cut into flat, long boards that are treated and smoothed for construction purposes. Contractors purchase rough lumber when they need to customize its dimensions for unusual projects.