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The Avoirdupois System
The everyday weight system, from a grain of barley to a ton of coal.
Origin
Adopted for English commerce from the late thirteenth century, and made statutory by the Weights and Measures Act of 1824.
The catch
The hundredweight does not contain a hundred pounds. It contains 112, so that it divides evenly into eight stone.
The name comes from the Old French avoir de pois, goods of weight: that is, things sold by weight rather than by count. It coexisted for centuries with troy weight for precious metals and the apothecaries’ system for medicine, all three sharing the same grain but diverging immediately above it.