The paper's grain can be one of the most important aspects in acheiving a perfect fold. The direction of the grain has much to do with the overall quality of the fold. Paper that is folded parallel with the grain of the paper will fold much more cleanly, resulting in a crisp, straight fold. Parallel grain produces a much cleaner fold because the paper fibers (grain) are running in the same basic direction as the fold. Only a few of the fibers run in the opposite direction and are able to provide any resistance to the folding mechanism, thus resulting in a precise, high quality fold.
When a fold is applied perpendicular to the grain of the paper, the resulting fold may have a ragged appearance. Perpendicular grain produces a ragged fold because all of the fibers are folded at one time creating resistance to the folding action. The ragged appearance of the fold is especially noticeable when folding heavier stocks of paper. More fiber results in more resistence, and lower quality folds.