The diagonal line or 45 degree angle of lengthwise and crosswise grain in fabric is termed as true bias. Following are the uses of bias in garment construction:
The fabric shows maximum stretch at true bias and so can either stretch or contract, thereby helping to finish curved edges neatly. Curved edges like necklines, armholes, scallops etc can only be finished with bias strips. Straight pieces cannot be used to finish the curved edges as it will give a bulky and unsightly finish.
Contrast colored bias strips can be used to give a decorative finish to necklines, hems etc.