Checking grain direction is an important requirement for papercrafters. I outline several reliable methods elsewhere [127784], but they do take a few moments.
This is a quick and easy method of finding grain direction ~ it has a fifty-fifty chance of being correct. Here are two methods that may well lead to trouble.
It is often fair to assume that grain runs parallel with the long side of the sheet. It does not always. I have seen paper catalogues where the helpful compiler has listed 'A3 {short gain}' since some paper mills do not cut A3 as standard, rather it is prepared from half an A2 {long grain}. Bookbinders ~ if given the choice ~ will prepare their folios from short grain sheets. We sell such papers in our shop.
I think it fair to say that all office and supermarket A4 papers are long grain. To halve such sheets and then use the resulting A5 sheets as folios to make an A6P (portrait) booklet is a good idea ~ and one of the reasons why I always use that size as a starter in my bookbinding classes. The fold runs with the grain. For a book to be bound 'properly' the grain of every element ~ folios ~ cover boards ~ end papers ~ covering papers must run parallel to the spine. It doesn't have to ~ but why make things difficult by breaking with the sensible and long established traditional requirement?
Rolled sheets tend to take up a slight curl ~ as does dampened paper. It pays to reflect on why sheets are rolled in the first place. Mass produced papers are made at high speed and the quickest and easiest method of collecting the finished product is to roll it into a cylinder. These rolls have the grain running the along the length of roll. A roll of lavatory paper is one example. To tear a sheet away from the roll would be difficult. The manufacturer embeds perforations to make it easy. An example I use in bookcraft class is a till-roll. Trying to tear a length of these is hopeless. I would suggest tryingit out ~ except till rolls are not very commonplace. Shopkeepers have no trouble ~ because the printing machine has a built in device for cutting the roll.
If you heed the above paragraph then you will beable to ascertain grain direction accurately ~ provided you are aware of one exception. I go back to 'Why are sheets rolled?' They are rolled for transit. To make the package as small as possible sheets tend to be rolled so the resulting the cylinder is as norrow as possible. A sheet printed or cut to be long grain ~ that is ~ the grain runs parallel with the length ~ will probably be rolled against the grain. If it rolled for a long time it will have a semi-permanent curl. This curl is likely to be cross grain. Check by using some other test to be certain.
This is a reliable method when dealing with laid papers ~ providing you can identify the chain marks. Hold a sheet of laid paper up to the light and you will see two sets of marks at right angles. Those that are close together are the laid marks or wire marks. These laid marks give the paper a slight rough texture, mainly on one side. Some printing porcesses give blotchy results if printed onto the rougher side.
As the paper is made, the pulp is dried and it is kept in place by a narrow grid of wires ~ through which the water can drain away. These run across the width of the paper roll ~ and therefore across the grain of the paper. The wires themselves are kept in place by another set of wires, more distantly placed. These also leave marks on the paper ~ the chain marks. To go with the grain follow the chain marks ~ not the laid marks. This is sometimes made more difficult since the laid marks are more numerous and often the easier to spot.