Grain direction
The direction most paper fibers run. Fold parallel to it for a clean bend; fold across it and thick stock can crack or spring open.
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What it is
Paper fibers line up mostly in the direction the web travels through the paper machine. A grain-long sheet has fibers running parallel to its long edge; grain-short runs parallel to the short edge. Paper bends more cleanly parallel to that grain and resists bending across it.
Put the grain parallel to the spine of a book and parallel to the main fold of a card, brochure, or carton. The pages open more willingly, the fold stays straighter, and heavy ink is less likely to split. This is why a book made cross-grain can feel stiff even when its layout and binding look correct.
Gotcha: scoring reduces surface cracking but does not erase a bad grain choice. Grain can also change when a parent sheet is cut down, so do not infer it from the finished dimensions. Put the required direction on the production spec and confirm it against the actual stock.
Ask AI for it
Build an Adobe InDesign production plan for this folded piece and mark the required paper grain with a labeled arrow on a nonprinting technical layer. Keep the grain parallel to the binding spine and the dominant fold, list every secondary fold that runs cross-grain, and add a score to each cross-grain fold through heavy ink. Include the grain requirement in the stock specification and require a folded physical dummy before approval.