A style line is a line or curve in a garment that has a visual effect, e.g., the seam between two fabrics of different colors or textures. For comparison, a nearly invisible seam, such as a dart or pleat, would not be considered a style line. A style line is a boundary between two distinguishable areas of fabric, or a visible edge of fabric such as the neckline, waistline or hemline.
As the term is generally used in practice, a style line is introduced only for fashion or cosmetic purposes. However, a style line may also fulfill a shaping function as a dart, a seam at which a dart may end, or as a way of hiding details of the garment's construction.