The pound (abbreviation: lb) is a unit of mass or weight in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system. The most commonly used pound today is the international avoirdupois pound. The international avoirdupois pound is equal to exactly grams. The definition of the international pound was agreed by the United States and countries of the Commonwealth of Nations in 1958. In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963. An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains.