Grain:

The direction of the filaments that make up paper or the most modern commercial paper will have a pronounced grain direction.

It is important to fold with the grain, not only is it easier, but will help in the longevity and sharpness of the fold as well as mitigate waves if the paper gets wet. There are several ways of determining the grain; the roll method is the least invasive. Hold the paper lengthwise with one hand on either end allowing the paper to sag. Put the sides together and observe the size of the "u" at the bottom. Repeat with the paper in the other direction. The direction that makes a narrower "u" is the direction of the grain. Most normal text paper has a grain that runs long (down the eleven inch direction).