Tun

The largest traditional English cask, holding 252 wine gallons, and the origin of a ship’s tonnage.

The tun sat at the head of the cask hierarchy: two butts, four hogsheads, or six tierces. Because medieval customs duty was levied on the number of tuns a vessel could carry, a ship came to be described by how many it could stow, and the count was recorded as its tunnage. Over centuries the spelling drifted to tonnage and the meaning shifted from a cask count to a measure of enclosed volume, but every modern vessel measured in gross tonnage is still, distantly, being described in barrels of wine.