The story behind the ounce is long and convoluted because people have been dissatisfied with the unit. For example, in medieval times, English merchants were not happy with the troy pound, as it was less than the commercial pound in most of Europe. In response, the merchants developed an even larger pound, called the libra mercatoria, or mercantile pound. But by 1300, the complaints about the mercantile pound grew, because 15 troy ounces (or 7,200 grains) was easily divided by 15 and its divisors, but not as convenient as dividing by 12 troy ounces.

Soon, another type of pound was born in English commerce: the 16-ounce avoirdupois (roughly translated from the Old French as “goods of weight”). Modeled on a common Italian pound unit of the late 13th century, the avoirdupois pound weighed exactly 7,000 grains, which is easily divided for use in sales and trade. But because it was difficult to convert between the troy and avoirdupois units (the avoirdupois ounce is 7,000/16, or 437.5 grains, and 1 grain equals 1/7,000 avoirdupois pound, or 1/5,760 troy or apothecaries’ pound; the troy ounce is 5,760/12,480 grains, or 31.1035 grams in metric), the standard soon shifted to using mostly the avoirdupois unit.

The avoirdupois is currently used in United States and Britain. It is equal to 1/16th of a pound or 28.3495 grams (in metric); the avoirdupois ounce is further divided into 16 drams (or drachm). The troy ounce hasn’t been totally forgotten, though. Today, it is used mainly as units for precious metals and drugs, where it is often called the apothecaries’ ounce (with its subdivisions of the scruple, or 20 grains and the drachm, or 60 grains). In turn, the avoirdupois—our “ounce” for short—is used for almost everything else.