According to the American Lumber Standards set by the National Bureau of Standards for the U.S. Department of Commerce, lumber is graded for quality. The major grades of yard lumber, in descending order of quality, are SELECT LUMBER and COMMON LUMBER. Each of these grades is subdivided, also in descending order of quality.

SELECT LUMBER has a good appearance and good qualities for finishing. One kind of select lumber is suitable for natural finishes; another kind, for painted finishes. Select lumber for natural finishes is graded A or B. Grade A is nearly free of defects and blemishes, but Grade B contains a few minor blemishes. Select lumber for painted finishes is Grade C or D. The blemishes in Grade C are more numerous and significant than those in Grade B. Grade D has even more blemishes than Grade C does. Either grade, C or D, presents a satisfactory appearance when painted.

COMMON LUMBER is suitable for general construction and utility purposes. It, also, is subdivided by grade in descending order of quality. No. 1 common is sound, tight-knotted stock, containing only a few minor defects. It must be suitable for use as watertight lumber.

Table 6-2.-Nominal and Dressed Sizes of Lumber

significant defects but no knotholes or other serious defects. It must be suitable for use as grain-tight lumber. No. 3 common contains a few defects, larger and coarser than those in No. 2; for example, occasional knotholes. No. 4 is low-quality material, contains serious defects like knotholes, checks, shakes, and decay. No. 5 common holds together only under ordinary handling.

STRUCTURAL LUMBER is graded accord-ing to allowable stresses that determine its safe No. 2 common contains a limited number of load-carrying capacity. This capacity is based on various factors, such as species of the wood, density, moisture content, and other character-istics that affect the strength of the lumber.

Factory and shop lumber is generally graded by its intended use; the grades vary greatly from use to use.