Before the introduction of the decimal metric system, standard weights were produced in a bewildering variety of shapes. In antiquity, geometrically shaped and artistically crafted pieces of metal or stone came into use. In more recent times, cylinder-shaped weights with knobs on top for easy handling were adopted as the most convenient design. To achieve ever greater accuracy, metals have become the material of choice, thanks to their consistently and universally recognized qualities: durability, inalterability, and hence capacity to provide greater precision. Prototypes (standard weights) made of brass, iron, and copper have been used extensively, but glass, stone, and ceramic have also been employed. In abstract terms, weight has been used since Isaac Newton (1642-1727) to refer to the force exerted on a body by gravitation.