A weighing scale (usually just "scale" in common usage; except in Australian English where "scales" is more common) is a measuring instrument for measuring the weight of an object (the gravitational attraction between the object an the Earth). They use one of two techniques. A spring scale measures weight by the distance a spring deflects under its load. A balance, described below, compares the unknown weight to a standard weight using a horizontal lever. Weighing scales are used in many industrial and commercial applications, and products from feathers to loaded tractor-trailers are sold by weight. Specialized medical scales and bathroom scales are used to measure the body weight of human beings.