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Lager (German: storage) is a type of beer that is fermented and conditioned at low temperatures. Pale lager is the most widely consumed and commercially available style of beer in the world. Bock, Pilsner and Märzen are all styles of lager. There are also dark lagers, such as Dunkel and Schwarzbier.
While cold storage of beer, "lagering", in caves for example, was a common practice throughout the medieval period, bottom-fermenting yeast seems to have emerged as a hybridization in the early fifteenth century. However, in 2011, an international team of researchers writing in the Proceedings of the National Academy of Sciences claimed to have discovered that Saccharomyces eubayanus, a yeast native to Patagonia, is responsible for creating the hybrid yeast used to make lager.
The rise of lager was entwined with the development of refrigeration, as refrigeration made it possible to brew lager year-round (brewing in the summer had previously been banned in many locations across Germany), and efficient refrigeration also made it possible to brew lager in more places and keep it cold until serving; the first large-scale refrigerated lagering tanks were developed for Gabriel Sedlmayr's Munich brewery by Carl von Linde in 1870. 
The average lager in worldwide production is a pale lager in the Dortmunder or Pilsner styles. The flavor of these lighter lagers is usually mild, and the producers often recommend that the beers be served refrigerated. However, the examples of lager beers produced worldwide vary greatly in flavour, colour, and composition.
In strength, lagers represent some of the world's most alcoholic beers. The very strongest lagers often fall into the German-originated doppelbock style, with the strongest of these, the commercially produced Samichlaus, reaching 14% ABV.
Lagers in certain countries often feature large proportions of adjuncts, usually rice or maize. Adjuncts entered United States brewing as a means of thinning out the body of U.S. beers, balancing the large quantities of protein introduced by six-row barley. Adjuncts are often used now in beermaking to introduce a large quantity of sugar, and thereby increase ABV, at a lower price than a formulation using an all-malt grain bill. There are however cases in which adjunct usage actually increases the cost of manufacture.
Pale lager is a very pale to golden-coloured lager with a well attenuated body and noble hop bitterness. The brewing process for this beer developed in the mid 19th century when Gabriel Sedlmayr took pale ale brewing techniques back to the Spaten Brewery in Germany and applied it to existing lagering brewing methods.
This approach was picked up by other brewers, most notably Josef Groll who produced in Bohemia (today Czech Republic) the first Pilsner beer - Pilsner Urquell. The resulting pale coloured, lean and stable beers were very successful and gradually spread around the globe to become the most common form of beer consumed in the world today.
The main elements of the lagering method used by Sedlmayr and Groll are still used today, and depend on a slow acting yeast that ferments at a low temperature while being stored. Indeed, the German term 'Lager' means 'storage'. While first marketed as 'Lagerbier' in Austria and Germany, the term is now quite uncommon in the German-speaking countries where today one would simply ask for 'helles Bier' (pale lager), 'dunkles Bier' (dark lager) or specific varieties, particularly those with a distinctive character such as Pilsner.
Lagers would likely have been mainly dark until the 1840s; pale lagers were not common until the later part of the 19th century when technological advances made them easier to produce. Dark lagers typically range in colour from amber to dark reddish brown, and may be termed Vienna, amber lager, dunkel, tmavé, schwarzbier, or Baltic porter depending on region, colour or brewing method.
Vienna or amber lager was developed by brewer Anton Dreher in Vienna in 1841. Austrian brewers who emigrated to Mexico in the late 19th century took the style with them. Vienna lager is a reddish-brown or copper-colored beer with medium body and slight malt sweetness. The malt aroma and flavor may have a toasted character. Despite their name, Vienna lagers are generally uncommon in continental Europe today but can be found frequently in North America, where it is often called pre-Prohibition style amber lager (often shortened to "pre-Prohibition lager"), as the style was popular in pre-1919 America. Notable examples include Great Lakes Eliot Ness, Samuel Adams Boston Lager, Devils Backbone Vienna Lager, and Yuengling Traditional Lager. In Norway, the style has retained some of its former popularity, and is still brewed by most major breweries.
Tmavé is Czech for "dark", so is the term for a dark beer in the Czech Republic - beers which are so dark as to be black are termed černé pivo, "black beer". Dunkel is German for "dark", so is the term for a dark beer in Germany. With alcohol concentrations of 4.5% to 6% by volume, dunkels are weaker than Doppelbocks, another traditional dark Bavarian beer. Dunkels were the original style of the Bavarian villages and countryside. Schwarzbier, a much darker, almost black beer with a chocolate or liquorice-like flavour, similar to stout, is mainly brewed in Saxony and Thuringia.