Down is the layer of fine, soft feathers found underneath the exterior feathers of some birds. Down is used as a natural thermal insulator and padding in a wide range of merchandise such as winter jackets, sleeping bags, footwear, and bedding. Down is lightweight and can be packed tightly into small spaces making ideal for outdoor gear. The loose structure of down feathers traps body heat which helps to insulate the body against the cold, while providing excellent breathability. The quilted design found in most down garments helps keep the insulating material evenly distributed inside the garment. Although natural down is the main material used in high quality cold weather garments, some manufacturers produce down alternatives made from synthetic materials such as polyester which is used in more affordable merchandise.