Grain refers to the direction in which the paper fibers are laid down during the manufacture of a sheet of paper. Determining the grain of all the elements you use to make a book – the paper, book board and book cloth – is important for creating well made books. The direction of the fibers in all these elements must run parallel to the spine.
There are three quick ways to figure out the grain direction for paper:
Gently bend the paper in both directions to see which yields more easily. The paper fibers run in the direction that gives the least resistance.
Fold and crease the paper first in one direction and then the other. If the paper fibers crack and break, you are folding against the grain. If the fold is smooth, you are folding with the grain.
Lightly spritz the paper with water and it will curl in the direction of the grain.
If you would like to learn more about paper grain, watch this video I created: