libra mercatoria [Latin]

In England, 13th? – 15th century, a unit of mass, used for domestic trade in heavy goods. Also called the London pound and, by authors writing after it was no longer in use, the commercial pound. Like the domestic pounds of France and Scotland at that time, it was a 15-ounce pound. Each ounce originally consisted of 450 grains (20 pennies × 22½ grains), so the pound was 6750 grains, or about 437 grams.²

The libra mercatoria was displaced by the 16-ounce pound avoirdupois in the 1400's.

The French 15-ounce pound, called the livre grosse, is mentioned in an ordinance of Philipe le Bel in 1307. The ounces were ounces of the marc of Paris. During the reign of Charles VI (1384-1387), the number of ounces was ordered raised to 16, making the livre equal to 2 marcs. This livre took the name livre poids de marc = about 489.5 g.

1. The name comes from Fleta:

Item denarius sterlingus, sicut dictum est, ponderate xxxij. grana frumenti, et pondus xx. d. facit unciam, et quindecim uncie faciunt libram mercatoriam.

Item: the penny sterling, as the saying is, weighing 32 grains of wheat, and a weight of 20 pennies makes an ounce, and 15 ounces make the libra mercatoria.

Fleta, writing in about 1290.
H.G. Richardson and G. O. Sayles, editors and translators.
Fleta. Vol. II, Book 2.
Selden Society Publication Vol. 72 of 1953.
London: The Selden Society, 1955.
Page 119.

2. We follow Connor and Simpson:

A. D. C. Simpson and R. D. Connor.
Weighing in the early 14th century.
Equilibrium, (1996), issue no. 1, pages 1987-1998; issue no. 2, pages 2015-2024.

A. D. C. Simpson and R. D. Connor.
Fourteenth Century Weight Systems: A Response.
Equiibrium, (1997), issue no. 1, pages 2107-2110.

Connor and Simpson, Weights and Measures in Scotland. (2004)

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