Double layer bias binding is a unique yet very simple binding style that allows you to skip a few folding and stitching steps in the application process.
Just like any binding style, double layer binding is cut on the bias for optimal stretch and flexibility, especially when used to finish curved and irregular-shaped fabric edges. What makes it unique however, is that it is cut 6 times wider than the desired finished binding application, and folded once down the center lengthwise to provide a fold edge. This folded edge provides a convenient shortcut in the sewing process as it serves as a pre-finished edge which can be left visible along the garment's edges.
Double layer binding, while convenient and simple to master, does not work with all fabrics as it tends to cause added bulk and thickness. It is best to use double layer binding with lightweight, especially sheer fabrics that will not be weighed down or overwhelmed by the additional binding layers. In addition, when used with fine, flimsy fabrics, double layer binding is easier to work with since the doubled bias strip provides more stability and comfort in the application process.
Double layer binding is aligned and stitched to a garment's raw edge such that all fabric raw edges are aligned. In this position, the binding pre-folded edge is brought over all fabric raw edges to encase them for a clean finish. As opposed to single layer biding's, double layer binding is only folded twice which allows for a simpler, more intuitive application process. To permanently encase all raw edges, double layer binding is slipstitched through its pre-folded edge and garment layer underneath. Slipstitching provides a clean, professional finish with no visible stitching on both the right and wrong side of the garment. If suitable for your design, you may choose to topstitch the binding closed by machine - this will leave a visible machine stitch along the binding's folded edges.