tunna

Tunna is Medieval Latin for a large cask or barrel. The word may come from Old English tunne or from a Celtic source — both claimed the word early. A tun was the largest standard wine barrel, holding about 252 gallons. When merchants needed to describe the weight of a shipload, they counted barrels. A ship that could carry twenty tuns was a twenty-tun ship. The weight of the barrel became the unit. The container became the count.

The word tun evolved into ton in English, and its weight varied by era and commodity. The English long ton settled at 2,240 pounds (1,016 kg) — twenty hundredweight of 112 pounds each. The American short ton is 2,000 pounds (907 kg) — twenty hundredweight of 100 pounds each. Neither matches the metric tonne, which is exactly 1,000 kilograms (2,204.6 pounds). Three tons, three weights, one word.

The metric tonne was introduced with the metric system in France in the 1790s. The French chose the familiar word tonne and defined it decimally: 1,000 kilograms. The spelling 'tonne' (with an e) was adopted in English to distinguish the metric unit from the English ton. In practice, the distinction is imperfect. News reports use 'ton' and 'tonne' interchangeably, and the difference between 2,000 pounds, 2,240 pounds, and 2,204.6 pounds disappears in casual use.

Shipping still uses tonnage — a word that literally means 'how many tun-barrels can the ship hold.' Gross tonnage, net tonnage, deadweight tonnage — all measure a ship's capacity, and all descend from the medieval practice of counting barrels. The barrels are gone. The ships are containerized. But the word for a barrel is still the word for a unit of weight, and it still measures what a ship can carry.