The grain on paperboard refers to the direction in which the fibers in the paperboard are aligned. When paperboard is manufactured, the fibers are typically aligned in one direction, either the machine direction (MD) or the cross direction (CD).
Grain is important because it affects the way board folds, tears, and prints. Paperboard that is folded with the grain will fold more easily and neatly than board that is folded against the grain. Board that is torn with the grain will tear more cleanly than board that is torn against the grain.
When choosing board for a project, it is important to consider the grain direction. If you are folding the board, you will want to choose board with the grain in the desired direction.
The grain direction is also sometimes indicated by underlining the longer dimension of the paper. For example, a piece of board that is 11" x 17" would be grain long if the 17" is underlined.
It's important to consider the grain direction when designing and manufacturing paperboard products to ensure the best possible results. In general, it is recommended to fold or score paperboard in the direction of the grain to avoid issues with cracking or tearing.