The structure of paper, machine made, results in the greater proportion of the fibers in the formed sheet lying in the direction of the flow of the stuff. This determines what is called the “grain” of the paper. When paper is in the roll the grain of course is lengthwise of the web, but in the sheet the cutting and slitting may be arranged so as to leave the grain either lengthwise or crosswise of the sheet. This is an important consideration for a number of reasons. In the first place, it is easier to tear the paper with the grain than across, as the fibers are parted rather than fractured in this way. This is a point which might be utilized by printers when printing detachable coupons. Perhaps the most important consideration is the great difference in folding qualities. Many a paper will fold very nicely with the grain and crack badly if folded the other way. Again, a great difference is noticeable in the flexibility of books, dependent largely on whether the grain runs parallel or at right angles to the binding. If flexibility is desired, the grain should run parallel to the back of the binding. Occasionally a wide-paged pamphlet, especially of light-weight paper, is improved by the rigidity to be gained from having the fibers run at right angles to the binding. It is also true that this increases the strength of the binding, as the sewing or wire stitching passes around more fibers than if the grain ran up and down the page. Not infrequently does the middle signature of a pamphlet pull loose from the binding. Usually in such cases the paper is not strong anyway, but it could have had more resistance had the grain run at right angles to the binding. The tensile strength of a strip of paper is greater with the grain, but its elasticity is greater across the grain. A convenient way to ascertain the direction of the grain in papers that do not show it clearly by folding is to cut two narrow strips a few inches long, hold them by one end so that they coincide. When held horizontally, if the loose ends do not part, it indicates that the lower paper has its grain in the long dimension. If the lower paper has its grain crosswise, the loose end will sag away from the top strip, because, as above remarked, a paper is more flexible across the grain. This test may be applied either to sized or unsized papers. Another test is to cut a small square and moisten one side; the paper will curl into a little cylinder and the grain runs parallel to the length of the cylinder. This test cannot be applied to an unsized paper. This leads us to a consideration of the effects of moisture and humidity on paper.