What is grain direction?
This is a property of paper that affects the way it folds and the way it stretches when wet.
Tear a piece of newspaper one way and it tears straight but turn it 90 degrees and the tear will be ragged.
All machine made paper and card will have a grain direction: fibers tend to line up in one direction.
Try to make your books with the grain direction of all materials parallel to the spine of the book because it will help the book to open well and stay flat when closed.
Test the paper for grain direction to find the easiest way to fold it. The image shows the ‘hangover’ test for grain direction. Take two sheets of the same paper and rotate one of them 90 degrees. Hang the same amount over the edge of the desk to see which one drops the furthest. This will show the easiest way to fold the sheet.