Paper tends to tear at an angle so most tears will have a top and a bottom. If the tear passes through the text or an illustration, it is easy to see which is the top or bottom because the bottom of the tear will show the white paper fibers. If the tear does not pass through text, look at the tear very carefully before pasting it together.
Some tears will go with the grain of the paper while others go against the grain. Tears that go with the grain of the paper will usually be smooth and straight while tears that go against the grain will tend to have more feathered edges and will curve as they try to align themselves with the paper grain.
In simple paper tears the page has been torn one time and the tear has an obvious top and bottom. This is easy to see when the tear passes through the text or illustrations.

Finding the Grain
Complex tears are really more than one tear. When a torn page is not repaired, the page can easily tear again and the second tear can have a different top and bottom. Always look at the tear and lay the edges in position before applying adhesive to make sure the tear is in the correct position. If the edges are not in the correct position, the repair will not lie flat and the text or illustration may be obscured.

There are three ways to repair paper tears in book repair – I will cover these in individual posts: