There are three basic types of feathers, each with its own set of specific functions. Contour feathers line the wings, tails, and backs of birds, giving them a defined shape and acting as aerodynamic structures. Beneath them, lie the soft, fluffy feathers commonly referred to as down. As anyone who has ever used a down comforter or sleeping bag should know, down feathers are excellent insulators. Thus, their primary function is to maintain the proper body temperature of birds, a task that they are aided in by hair feathers. Sometimes alternatively known as filoplumes, hair feathers are also believed to function as pressure and vibration receptors that sense the location of other feathers so that they can be appropriately adjusted.