Butter is essentially the fat of milk. It is usually made from sweet cream (that is, cream skimmed from milk rather than whey). In the USA, Ireland, the UK and the Nordic countries, salt is usually added to it. Unsalted (sweet) butters are most commonly used in the rest of Europe. However, it can also be made from acidulated or bacteriologically soured cream. Well into the 19th century butter was still made from cream that had been allowed to stand and sour naturally. The cream was then skimmed from the top of the milk and poured into a wooden tub.