The art of brewing, which became highly developed in most English monasteries, can be traced back to Roman times. For two thousand years beers have been produced and stored in wooden vessels which have been lined with a variety of materials such as pitch to help seal them against leakage.
Originally, beer was brewed to meet the needs of small communities and was consumed at the production site but, as demand for it grew and beer had to be taken to more distant points-of-sale, transportable casks were required and these, too, were made from wood. The most common size of cask held one "barrel", a brewing unit which, in Medieval times, was a volume of 145.5 liters (32 Imperial gallons) but which is now standardized at 163.7 liters (36 Imperial gallons).
This volume naturally gave its name to the cask of that capacity, but was eventually adopted colloquially for all sizes of cask, despite their having their own names; the 4.5-gallon "pin", the 9-gallon "firkin", the 18-gallon "kilderkin" and the 54-gallon "hogshead" (20.5, 40.9, 81.8 and 245.5 liters respectively). Wooden casks were made from vertical strips of oak, or "staves", held tightly together by horizontal steel hoops(6). For this arrangement to be watertight, the staves were not only tapered so that together they created a circular cross-section, but also bowed so that steel hoops could be forced down from the circular end to squeeze them together. This gave rise to the bellied shape of casks, which offered the practical advantages that even the hogshead, which weighed nearly one third of a ton (700 kg) when full, could easily be rolled and steered along the ground with a stick or by gentle kicking.
Then, when it needed to be lifted up to, and laid horizontally on, the rack (or, "stillage") in the customer's cellar, the bellied shape allowed the container to be rocked backwards and forwards longitudinally until it could be lifted smoothly onto its end and then swung completely over and onto the stillage . There it was stored until the natural conditioning processes were complete and the beer was ready for drinking. The belly also retained the yeast sediment which settled during conditioning such that, even as the level in the cask fell, the beer was constantly drawn off from above the sediment, keeping it clear or, "bright."