Grain decides the fold before the design does
A beautiful cover folded across the grain will bow, crack or both.
A fold is a material test
Paper bends easily along the grain and resists bending across it. A fold across the grain puts stress on the fibre and the coating. On a coated cover the result is a cracked spine; on an uncoated text sheet it is a wavy page that never settles.
Where the decision is made
Grain is fixed when the paper is ordered. If the sheet is cut to fit the press, the mill grain may end up across the short side. That is why the grain axis belongs on the layout and on the purchase order, not only in a conversation with the printer.
The check to keep
Fold a blank of the actual stock in both directions before approving the layout. If the cross-grain fold cracks, change the sheet or add a score. Do not wait for the bound sample to discover it.