What is the point of a GRAINLINE
A Grainline on a sewing pattern is there to make sure the sewer cuts the fabric correctly so the garment "hangs" well (and not lopsided).
The grainline on a pattern piece determines the direction on the fabric the pattern should be cut.
When putting the pattern pieces on top of the fabric, locate the grainline and line the grainline up the same distance of the complete grainline measured parallel to the edge of the fabric (the selvage). Doing this before you cut the fabric ensures that your pattern piece follows the direction of the fabric's weave, and because of that, none of the pattern pieces are cut on the bias which is what will mess up the final product of the garment.