A style line is the seam where two different types of fabric meet in a piece of clothing. Sometimes called a garment line, a style line can also refer to edges of the clothing. The hem and the neckline do not join different fabrics in most cases, but their shapes affect the overall style of the clothing. There is a large difference between a garment line and an ordinary seam. Seams are usually meant to be hidden, and are generally only necessary to piece the garment together. A style line, on the other hand, is more noticeable.
Designers use this type of line with care because it is a dividing line between two different looks. A straight line gives a different appearance than a line along a garment curve. A seam that zigzags between two different colors will also give the garment a different look than a straight or curved line. If one of the pieces of fabric is gathered or pleated, the style line will probably be even more prominent.
Any type of seam between fabrics can be a style line if it is used to join different colors or patterns, but many seams rarely do. A dart, for instance, is usually not a considered a garment line. A dart is a seam that is invisible and is used to take up fabric and help shape a garment. A style line can serve as a dart, however, if the fabric on each side of the seam is different. A pleat is a fold of fabric that is sewn at the top, but is not considered a style line because it usually involves the same piece of fabric.
Waistlines are typically considered simple seams unless they are used for effect. An empire silhouette design, for instance, has a high waist that ends in a seam just below the bust. A skirt often flows from that seam. If the materials used for both the top and the skirt are the same, the waistline is a simple seam. If the fabrics are different, the waistline becomes a style line.
One garment can have several style lines. Sleeves, collars, pockets, and trim are sometimes done in different fabrics for effect. The seams between those pieces and the main garment would usually be considered style lines. The main piece of clothing can also be made up of many different colors and patterns. The pieces can be cut in different shapes to create a specific look or style.
When sewing, it is important to take care with this line because it will be so much more visible than any other seams. An imperfect seam between two pieces of the same material will usually be far less noticeable than an imperfect seam between two different types of fabric. Even if the seam is true, if one of the pieces of fabric bunches at the seam, it is likely to be noticed more on a style line than an ordinary seam.