Trade weights

The basic traditional unit of weight, the pound, originated as a Roman unit and was used throughout the Roman Empire. The Roman pound was divided into 12 ounces, but many European merchants preferred to use a larger pound of 16 ounces, perhaps because a 16-ounce pound is conveniently divided into halves, quarters, or eighths. During the Middle Ages there were many different pound standards in use, some of 12 ounces and some of 16. The use of these weight units naturally followed trade routes, since merchants trading along a certain route had to be familiar with the units used at both ends of the trip.

In traditional English law the various pound weights are related by stating all of them as multiples of the grain, which was originally the weight of a single barleycorn. Thus barleycorns are at the origin of both weight and distance units in the English system.

The oldest English weight system has been used since the time of the Saxon kings. It is based on the 12-ounce troy pound, which provided the basis on which coins were minted and gold and silver were weighed. Since Roman coins were still in circulation in Saxon times, the troy system was designed to model the Roman system directly. The troy pound weighs 5760 grains, and the ounces weigh 480 grains. Twenty pennies weighed an ounce, and therefore a pennyweight is 480/20 = 24 grains. The troy system continued to be used by jewelers and also by druggists until the nineteenth century. Even today gold and silver prices are quoted by the troy ounce in financial markets everywhere.

Since the troy pound was smaller than the commercial pound units used in most of Europe, medieval English merchants often used a larger pound called the "mercantile" pound (libra mercatoria). This unit contained 15 troy ounces, so it weighed 7200 grains. This unit seemed about the right size to merchants, but its division into 15 parts, rather than 12 or 16, was very inconvenient. Around 1300 the mercantile pound was replaced in English commerce by the 16-ounce avoirdupois pound. This is the pound unit still in common use in the U.S. and Britain. Modeled on a common Italian pound unit of the late thirteenth century, the avoirdupois pound weighs exactly 7000 grains. The avoirdupois ounce, 1/16 pound, is divided further into 16 drams.

Unfortunately, the two English ounce units don't agree: the avoirdupois ounce is 7000/16 = 437.5 grains while the troy ounce is 5760/12 = 480 grains. Conversion between troy and avoirdupois units is so awkward, no one wanted to do it. The troy system quickly became highly specialized, used only for precious metals and for pharmaceuticals, while the avoirdupois pound was used for everything else.

Since at least 1400 a standard weight unit in Britain has been the hundredweight, which is equal to 112 avoirdupois pounds rather than 100. There were very good reasons for the odd size of this "hundred": 112 pounds made the hundredweight equivalent for most purposes with competing units of other countries, especially the German zentner and the French quintal. Furthermore, 112 is a multiple of 16, so the British hundredweight can be divided conveniently into 4 quarters of 28 pounds, 8 stone of 14 pounds, or 16 cloves of 7 pounds each. The ton, originally a unit of wine measure, was defined to equal 20 hundredweight or 2240 pounds.

During the nineteenth century, an unfortunate disagreement arose between British and Americans concerning the larger weight units. Americans, not very impressed with the history of the British units, redefined the hundredweight to equal exactly 100 pounds. The definition of the ton as 20 hundredweight made the disagreement carry over to the size of the ton: the British "long" ton remained at 2240 pounds while the American "short" ton became exactly 2000 pounds. (The American hundredweight became so popular in commerce that British merchants decided they needed a name for it; they called it the cental.) Today, most international shipments are reckoned in metric tons, which, coincidentally, are rather close in weight to the British long ton.

Medieval balance scale arms