A grainline is a double-headed arrow found on all sewing patterns. Grainlines have the important task of denoting the garment's desired grain thus providing the patterns' correct placement on the fabric during the alignment and cutting process. A lengthwise grainline is positioned vertically on a sewing pattern. Vertical means from top (neckline) to bottom (hemline).
In the pattern cutting process, grainlines are positioned parallel to the fabric selvage edge. A lengthwise grain follows the fabric's warp threads. The warp threads are those moving parallel to the selvage edge.
Almost all garments are cut on a lengthwise grain. The fabric's lengthwise grain provides the most desirable drape and durability in the finished garment. Given that this grain is parallel to the fabric's selvage, lengthwise alignment allows for any desired garment length especially when working with floor-length styles. Keep in mind that many fabrics are intentionally printed and woven to work best when the pattern pieces are cut on the lengthwise grain.