You might have noticed that when you tear a square from a sheet of paper, one direction leaves a nice clean edge but the edge is much more ragged on the right-angled tear. You’ll have also spotted that when folding A4 sheets in to A5 booklets there’s a strong tendency that the booklet will spring open. Paper displays different qualities depending on whether you’re working in the direction of the ‘grain’ or ‘across the grain’.
Because paper is made from long fibres of one form or another, there is a tendency for paper machines to draw these fibres in to the machine in a particular direction. Naturally papermakers look to avoid this but even in fine papers you’ll end up with long fibres running in the same direction as the paper machine runs. This is the ‘grain direction’. There’s no firm rule about which direction the grain will run in any given package, but paper merchants tend to cut the paper so that the grain runs from top to bottom. On larger sheets that you might buy directly the grain could run in either direction.
There are two simple tests to find grain direction. If you tear the paper once and again at right-angles, you’ll see that the tear looks different: with the grain will leave a much cleaner tear; across the grain will leave a more messy tear. Alternatively you can cut two similar pieces of paper around 2″ x 6″ at right angles to each other and put them together. Holding one end out you’ll find that one might flop and another seem more strong; if not one might be ‘supporting’ the other. This ‘supporting’ or stronger piece will have the grain running down the length of the paper.
Grain direction will affect tensile strength; rigidity and stability. The degree to which these affect us will depend on the specific paper and maker so there are no concrete rules on how big a problem this might be for you.
Strength will be most important in some specialist applications: bottle labels, for example, are applied left-to-right on the bottle and so are pulled from left-to-right by the labelling machine. Because the paper will be strongest with the grain, it should be printed so that the grain runs from left-to-right. Other areas might include hole punched documents that might be subject to pulling from left-to-right.
To prevent ‘springing’ when a booklet is opened; the grain should run parallel with the spine. Look out for this if you intend to print, say, an A5 booklet from A4 because the A4 is usually sold with the grain running from top-to-bottom. Using this will lead to the folded booklet springing open.
Rigidity is important where printed articles have to be handled or stored upright. Index cards, for example, need to be printed with the grain running from top-to-bottom as this will mean the card is strongest in this direction, and so avoid the cards flopping down or curling slightly. Clocking-in cards should be printed with the grain running top to bottom to help with the handling of the cards.
Finally, stability rests on the tendency of paper to take up water from its surroundings. When this happens the paper swells across the grain because fibres have a tendency to expand in their width rather than length. This is most important when doing very close register work or colour printing. There may be a direction where the register is less important. One example would be ledger sheets where top-to-bottom register is less important than the need to get left-to-right columns correct, for a change in these could lead to mistaken use of the wrong column. In this case, if the tolerances are very tight, consideration should be given to have the grain running left-to-right across the page so that any expansion in the fibres has an effect top-to-bottom rather than left-to-right.
All of these considerations seem small in comparison with machining or the initial layout but it’s these small details that will make the final job look and feel better than one without consideration of the grain direction.