Grainline
A line on a pattern piece that shows how it must be placed on the fabric, parallel to the direction of the threads.
Woven fabric has lengthwise threads (the warp) and crosswise threads (the weft). The grainline normally follows the lengthwise threads, parallel to the selvage, which is the most stable direction and the one that hangs straight; in knit fabric it follows the wales. A piece cut off-grain can twist, stretch or hang crooked after washing. It is drawn as an arrow and, in a marker, it must be kept parallel to the edge of the fabric within the tolerance allowed.
Example
On the front piece of a skirt, the grainline arrow runs from the waist to the hem; when the piece is laid on the fabric, the arrow is parallel to the selvage.