were not happy with the troy pound, as it
was less than the commercial pound in
most of Europe. In response, the mer-
chants developed an even larger pound,
which was called the libra mercatoria,or
mercantile pound. But by 1300 the com-
plaints about the mercantile pound grew,
because 15 troy ounces (or 7,200 grains)
were easily divided by 15 and its divisors,
but this was 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”). Mod-
eled 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 con-
vert 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 met-
ric), the standard soon shifted to using mostly the avoirdupois unit.
The avoirdupois is currently used in the 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.What were some early measurements of volumein terms of the gallon?
The names of the traditional volume units are the names of standard containers. Until
the 18th century, the capacity of a container was difficult to accurately measure in
cubic units. Thus, the standard containers were defined by the weight of a particular
substance—such as wheat or beer—that the container could carry. For example, the
basic English unit of volume, or the gallon, was originally defined as the volume of
eight pounds of wheat. Other volumes were measured based on this gallon, depending
48 on the different standard sizes of the containers.
Gold bars are still measured using old-fashioned troy
ounces. Thus, a pound of gold is equal to 12 troy
ounces, not the usual 16 ounces per pound people
typically use to measure other weights. The Image
Bank/Getty Images.