Bias grainlines are located diagonally on a sewing pattern. A bias grainline extends from a top corner to the opposite diagonal bottom corner.
A pattern's bias grainline follows the fabric's bias grain. The fabric's true bias is positioned at a 45 degree angle in relation to the fabric's horizontal and vertical edges. To find the fabric's most accurate bias, fold the fabric such that the crosswise edge is parallel to the selvage. Doing so will form a diagonal fold that corresponds directly to the fabric's true bias. If needed, you can iron this fold to mark the bias line- this is often done when cutting bias binding at home.
A garment is cut on the bias when added stretch and a higher drape is desired. Since a bias cut requires more fabric, it is a considered a more expensive dressmaking technique.
Woven fabrics have significant stretch across the bias thus when the garment pieces are cut on the bias grain, they behave more like knits and should be handled with care. Bias cut edges risk stretching out of shape which can often make aligning and sewing the seams more complicated. Nevertheless, bias-cut garments result in a fluid, comfortable drape often used with long dresses/skirts and evening gowns.
In the cutting process, pattern grainlines are always positioned parallel to the fabric's selvage edge regardless of whether they are crosswise, lengthwise or bias. In this case, the bias grain alignment will use up more fabric and take up more space and thus it is important to consider these elements when deciding how much fabric to purchase and where/how to set up your work space. Keep in mind that bias edges can stretch out of shape so you should handle all pieces with care once they are cut and separated from the corresponding patterns.