m = mass g = local gravitational acceleration (on earth about 9.8 m/sec/sec; ,depending on exact location and altitude) The force W is commonly referred to as weight. Because gravitational acceleration depends on a particular location, weight also depends on location. For instance, if an object is transported to the moon -with its lower field of gravity- the mass of this object remains constant even though its weight changes. Force (or weight), and mass must not be confused: Force is the effect on an object that deforms or accelerates it (e.g. during free fall). Mass is a property of an object, describing its quantity. There is an additional difference between force and mass: To describe a force, its size and direction must be defined. For defining mass the size suffices. The units for mass and force are likewise different: Mass is measured in kilogram (abbreviated kg), force is measured in newton (abbreviated N). where 1 N = 1 kg . 1 m/sec/sec. The kilogram is a base unit of the International System of Units. It is the only base unit that is not derived from natural constants: The kilogram is defined by the mass of a prototype. This international kilogram prototype is kept at the “Bureau International des Poids et Mesures”, BIPM, at Sèvres near Paris. The National kilogram prototypes are compared with the international kilogram prototype by weighing at the BIPM. The primary standards of national metrological institutions are tested against the national prototype, and are used, in turn, to test the secondary standards of verification boards. The result is a hierarchy of mass standards in which the accuracy is lower on each level, but the frequency of use is greater than that of the preceding level. (see table below)