Grainline

Grainline refers to the direction of the lengthwise (warp) threads in a woven fabric — the threads that run parallel to the selvage. Nearly every pattern piece has a printed grainline arrow, and matching that arrow to the fabric's lengthwise grain when you lay out your pattern is what makes a finished garment hang the way it's supposed to.

Cut a piece off-grain and the fabric will twist or lean once it's sewn and worn, even if every seam allowance is technically correct — I've had this happen on a pair of trousers early on, and no amount of pressing ever fully fixed the twist in the leg. It's a layout mistake, not a sewing mistake, so it can't really be corrected after the fact.

The exception, deliberately: bias-cut garments, where you rotate the pattern 45 degrees off the straight grain on purpose for extra drape and stretch. That's not a mistake, it's a whole different technique with its own logic — but everywhere else, the grainline arrow is not a suggestion.