Paper is made up of individual fibers from materials such as wood or cotton fiber. In the papermaking process on a Fourdrinier paper machine (the most common type), the fibers align themselves in the direction of the vibrating wire mesh upon which the paper is formed. This is called the machine direction or grain direction. As the fibers travel through the machine on a vibrating mesh, they tend to line up in the same forward direction, parallel to the direction of the web or side edges of the paper. This is called the grain direction.

The grain direction of the paper is important when binding books, because paper is scored and folded. For bookbinding, you should always fold with the grain direction parallel to the fold and the spine of your binding. Folding parallel to the grain is easier and the paper is less likely to “crack,” create a rough fold and result in a bulky binding edge. It also helps book’s pages to turn easier, stay open and lay flatter.

A paper is called “grain long” if the grain is parallel to the paper's long side. If you are uncertain of the grain direction, there are techniques you can use to determine the direction in papers. If the grain direction is strong enough, you can identify the direction by “feel.” Take the sheet of paper in your hands and gently curl/bow the paper towards the middle, first in one direction and then in the other direction. The direction that bends more easily, with less resistance, is the grain direction.