" There are considerable complexities attached to other measures. It is not so much that they vary as that they may be differently interpreted, according to what it is you are trying to measure. A foot in length is the same as your modern foot of 12 inches but if you are measuring cloth then you use the ell, normally 45 inches—but 27 inches if the cloth is Flemish. Probably the most complicated measures are those involving liquids. A gallon of wine is not the same volume as a gallon of ale. A standard hogshead contains 63 wine-gallons or 52.5 ale-gallons. Except that there is no such thing as a standard hogshead; there is a standard for wine, another for ale, and a third for beer (which is imported). If you are buying beer in London, a hogshead amounts to 54 ale-gallons; if you are buying ale, it amounts to 48. "

― Ian Mortimer , The Time Traveller's Guide to Medieval England: A Handbook for Visitors to the Fourteenth Century