A measure is the value of a physical quantity expressed as the ratio between that quantity and another quantity of the same kind, chosen as a standard and conventionally accepted as a unit (unit of measurement). Units of measurement may denote capacity, length, weight or time. Since the 19th century, and still today, specialized producers build and sell measuring instruments to merchants and consumers including scales, standard weights, yardsticks, and capacity measures. Every country makes periodic arrangements to inspect their accuracy either directly at the manufacturing plant or in stores. In fact, every country has specialized institutes in charge of checking the consistency of measures in circulation and acting to ensure their accuracy.