The direction you can't see
Paper is made from fibres suspended in water and laid onto a moving wire mesh. As the mesh pulls forward, most fibres align with the direction of travel. That alignment is the grain, and it runs the length of a commercial sheet in long-grain paper, across the width in short-grain. The sheet looks the same either way. The fold does not behave the same way at all.
Fold with the grain and the crease is clean and tight. Fold against it and the fibres resist, then fracture slightly along the surface — a rough, cracked edge visible on any heavy stock. On a wedding invitation, that crack runs right through the face of the type. On a folded order of service, it reads as carelessness. The heavier the paper, the worse the break: a 350gsm board against the grain is almost impossible to close without damage.
Weight — measured in grams per square metre, abbreviated gsm — controls how the piece sits and handles. A single card at 300gsm has presence; it doesn't curl in a warm hand. An insert sheet at 100gsm folds cleanly, fits an envelope without forcing it. Printers match weight to purpose before they think about texture or colour: soft-white uncoated stocks for letterpress printing, heavier coated boards for digital work that carries solid ink coverage without strike-through.
Testing grain takes ten seconds. Hold a sheet by one edge and let it hang; rotate it ninety degrees and hang it by the adjacent edge. The direction in which the sheet droops less is the grain direction. Alternatively, dampen one corner: the sheet will curl across the grain as the fibres swell unevenly. Both tests are done before cutting, not after.
Envelopes have their own grain requirements. The flap folds against the body, so the grain has to run in a direction that lets the fold close without the flap lifting or cracking — a problem the envelope handles in its own way.