Cask beer, also known as cask ale, is a beer that is unfiltered and unpasteurised. It goes through a secondary fermentation process and is served from a cask – or a container similar to a classic beer keg. Cask beer does not have additional nitrogen or carbon dioxide pressure, which is characteristic of most other beers. In the United Kingdom, cask ale is commonly referred to as “real ale”. A key difference in cask beer is the length of time the beer can be stored in the cask – and this is determined by a few different factors. Lighter beers needs to be put on tap and sold quickly, usually within a few days of tapping. Stronger, darker beers need extra time to settle and mature in the cask, and can last much longer and stored for months as long as the cask remains unopened. The length of time that these stronger cask beers need is up to the bartender who taps and untaps the product – so long as the cask remains sealed. Cask beer is most popularly found in pubs in Europe and the United Kingdom.