Paper is a material made by compacting fibers. During paper production, the fibers of paper pulp are mainly oriented in the direction of the paper machine's movement. Sheets are then cut from the resulting parent roll of paper according to a pattern outlined in a schematic diagram.
Archy, or sheets, cut from the parent roll are divided into sheets with a wide grain - BB and sheets with a narrow grain - SB based on the orientation of the fibers.
BB - Wide grain: Fibers are parallel to the short side of the sheet.
SB - Narrow grain: Fibers are parallel to the long side of the sheet.
Why is it important to know the grain direction of a sheet of paper?
The properties of paper vary with different grain directions. For example, paper is stronger along the grain direction when subjected to tension.
During printing, paper expands proportionally with moisture absorption. Along the grain direction, dimensional changes are less noticeable, while perpendicular to the grain direction, paper expands much more. This is important to know, especially in multi-color printing and color registration.
In bookbinding, it's crucial to respect the grain direction when folding paper. The grain must be oriented in the same direction as the fold; otherwise, the paper will break at the fold and unpleasantly crack along the spine of the cover.