Firkins

A quarter of a barrel equal to 9 imperial gallons, still utilised for cask ales.

The firkin is the working cask of British real ale and has been for centuries. Nine imperial gallons is seventy-two pints, which is roughly what a small pub can sell before the beer goes off: once a cask is broached and exposed to air, it has perhaps three days of life. That single constraint explains the whole cask hierarchy. A firkin is a quarter of a barrel, half a kilderkin and twice a pin, and the sizes exist so that a landlord can match the vessel to the rate of sale. The name descends from the Middle Dutch vierdekijn, a little fourth, which records its relationship to the barrel in the word itself. Confusingly, a firkin of butter is a weight of 56 pounds rather than a volume at all, which is a different trade using the same word.