Paper grain is the direction that most of the fibers in a piece of paper are pointed – it will either be toward the long end or toward the short end, not on the diagonal. It is important to know this in bookbinding, because, for optimal strength and to prevent warpage, the grain of the paper must be parallel with the spine of the book, and the grain of all the paper must go in the same direction.
For machine-made paper, the grain of the paper is perpendicular to the curve of the roll – parallel to the edge of the paper. For handmade paper, the direction of the grain can be found quite easily. Bend the paper gently lengthwise, then gently widthwise. The fold that folds more easily will be parallel to the grain of the paper. Paper also tears more easily when torn parallel to the grain, not perpendicular to the grain.
Paper grain is important because paper expands perpendicular to the grain when exposed to humidity. If the grain in the book block, endpapers, boards, and paper covering the boards is all facing the same direction, the expansion will only cause slight curving in one plane. If the grain is not all in the same direction, the expansion will be in multiple directions, causing warped boards that look like potato chips.