In the market, lumber is not only classified according to the above, but according to the species of tree it comes from, and also according to certain standard grades of the same kind. These grades are determined by the presence of certain defects. The recognized defects are knots, shakes, checks, splits, streaks, pitch-pockets, stain, rot, wane, warp, cupping, mineral streaks, pith on the face of the board, and worm-holes. Various large lumber associations issue rules governing standard sizes and classifications for woods to be used in construction. The best and the next best are the usual grades which are used for the interior and exterior trim of houses. These grades have many designations, as “clears” and “selects,” or “A” and “B,” or “No. 1” and “No. 2,” or “firsts” and “seconds.” The grades used for the rough framing, such as studs, joists, rafters, subfloors, and sheathing, are not so good. They are designated as “No. 1 common” and “No. 2 common.” A poorer grade still, known as “No. 3 common,” is sometimes used for cheap temporary structures. For the details of grading and standard sizes of lumber, one should possess Circular 64 of the United States Department of Agriculture on “How Lumber is Graded.” Next to the grading of timber, the most important factor of quality is the relative durability of the various woods, for upon this depends to a large extent the choice of them for special places. The table on page 23 is taken from a government classification. From this table it will be noticed that the soft woods as a class are relatively more durable than the hard woods. This is true, because of the fact that the structure of soft woods is simple, while that of the hard woods is complex. When the former become wet and expand and then dry out and shrink, the structure is not stressed internally as much as is that of the hard woods, and they are therefore much more capable of withstanding the action of the weather. Also certain of the soft woods have natural properties of resisting dry or wet rot. Certain species of woods are, therefore, selected for particular parts of the house according to the needs of durability, strength, appearance, and local supply. Rough wooden framing requires a wood that is fairly abundant and strong. The soft woods are generally used, and those which are classified as _durable_ in the table are the most used. RELATIVE DURABILITY OF THE COMMON WOODS