Stout is a dark, top-fermented beer with a number of variations, including dry stout, Baltic porter, milk stout, and imperial stout.

The first known use of the word stout for beer was in a document dated 1677 found in the Egerton Manuscripts, the sense being that a "stout beer" was a strong beer, not a dark beer. The name porter was first used in 1721 to describe a dark brown beer that had been made with roasted malts. Because of the huge popularity of porters, brewers made them in a variety of strengths. The stronger beers, typically 7% or 8% alcohol by volume (ABV), were called "stout porters", so the history and development of stout and porter are intertwined, and the term stout has become firmly associated with dark beer, rather than just strong beer.

Milk stout (also called sweet stout or cream stout) is a stout containing lactose, a sugar derived from milk. Because lactose cannot be fermented by beer yeast, it adds sweetness and body to the finished beer. Milk stout was claimed to be nutritious, and was given to nursing mothers, along with other stouts. Milk stout was also said to be prescribed by doctors to help nursing mothers increase their milk production. The classic surviving example of milk stout is Mackeson's, for which the original brewers advertised that "each pint contains the energising carbohydrates of 10 ounces [280 ml] of pure dairy milk".

It is widely reported that, in the period just after the Second World War when rationing was in place, the British government required brewers to remove the word "milk" from labels and adverts, and any imagery associated with milk. However, no specific legislation or orders have been found to support this, though there were some prosecutions in Newcastle upon Tyne in 1944 under the Food and Drugs Act 1938 regarding misleading labelling.

With milk or sweet stout becoming the dominant stout in the UK in the early 20th century, it was mainly in Ireland that the non-sweet or standard stout was being made. As standard stout has a dryer taste than the English and American sweet stouts, they came to be called dry stout or Irish stout to differentiate them from stouts with added lactose or oatmeal. This is the style that represents a typical stout to most people. The best selling stout is Guinness, made by Diageo at St. James's Gate Brewery (also known as the Guinness Brewery) in Dublin; Other examples of Irish dry stout include Murphy's and Beamish. Draught Irish stout is normally served nitrogenated, to create a creamy texture with a long-lasting head.

While there is a great deal of disagreement in the brewing world on this subject, there are no differences between stout and porter historically, though there has been a tendency for breweries to differentiate the strengths of their dark beers with the words "extra", "double" and "stout". The term stout was initially used to indicate a stronger porter than other porters issued by an individual brewery. Though not consistent, this is the usage that was most commonly employed. More recently, "stout" tends to be used to describe dry stouts, (containing a small amount of unmalted roast barley), or sweet stouts, (such as milk stouts), whilst "porter" describes a beer flavoured with roast malted barley.

Oatmeal stout is a stout with a proportion of oats, normally a maximum of 30%, added during the brewing process. Even though a larger proportion of oats in beer can lead to a bitter or astringent taste, during the medieval period in Europe, oats were a common ingredient in ale, and proportions up to 35% were standard. Despite some areas of Europe, such as Norway, still clinging to the use of oats in brewing until the early part of the 20th century, the practice had largely died out by the 16th century, so much so that in 1513 Tudor sailors refused to drink oat beer offered to them because of the bitter flavour.

There was a revival of interest in using oats during the end of the 19th century, when (supposedly) restorative, nourishing and invalid beers, such as the later milk stout, were popular, because of the association of porridge with health. Maclay of Alloa produced an Original Oatmalt Stout in 1895 which used 70% "oatmalt", and a 63/- Oatmeal Stout in 1909, which used 30% "flaked (porridge) oats".

In the 20th century many oatmeal stouts contained only a minimal amount of oats. For example, in 1936 Barclay Perkins Oatmeal Stout used only 0.5% oats. As the oatmeal stout was parti-gyled with their porter and standard stout, these two also contained the same proportion of oats. (Parti-gyle brewing involves blending the worts drawn from multiple mashes or sparges after the boil to produces beers of different gravities.)

The name seems to have been a marketing device more than anything else. In the 1920s and 1930s Whitbread's London Stout and Oatmeal Stout were identical, just packaged differently. The amount of oats Whitbread used was minimal, again around 0.5%. With such a small quantity of oats used, it could only have had little impact on the flavour or texture of these beers.

Many breweries were still brewing oatmeal stouts in the 1950s, for example Brickwoods in Portsmouth, Matthew Brown in Blackburn and Ushers in Trowbridge. When Michael Jackson mentioned the defunct Eldrige Pope "Oat Malt Stout" in his 1977 book The World Guide to Beer, oatmeal stout was no longer being made anywhere, but Charles Finkel, founder of Merchant du Vin, was curious enough to commission Samuel Smith to produce a version. Samuel Smith's Oatmeal Stout then became the template for other breweries' versions.

Oatmeal stouts do not usually taste specifically of oats. The smoothness of oatmeal stouts comes from the high content of proteins, lipids (includes fats and waxes), and gums imparted by the use of oats. The gums increase the viscosity and body adding to the sense of smoothness.

Oysters have had a long association with stout. When stouts were emerging in the 18th century, oysters were a commonplace food often served in public houses and taverns. By the 20th century, oyster beds were in decline, and stout had given way to pale ale. Ernest Barnes came up with the idea of combining oysters with stout using an oyster concentrate made by Thyrodone Development Ltd. in Bluff, New Zealand, where he was factory manager. It was first sold by the Dunedin Brewery Company in New Zealand in 1938, with the Hammerton Brewery in London, UK, beginning production using the same formula the following year. Hammerton Brewery was re-established in 2014 and is once again brewing an oyster stout.

Modern oyster stouts may be made with a handful of oysters in the barrel, hence the warning by one establishment, the Porterhouse Brewery in Dublin, that their award-winning Oyster Stout was not suitable for vegetarians. Others, such as Marston's Oyster Stout, use the name with the implication that the beer would be suitable for drinking with oysters.

Chocolate stout is a name brewers sometimes give to certain stouts having a noticeable dark chocolate flavour through the use of darker, more aromatic malt; particularly chocolate malt—a malt that has been roasted or kilned until it acquires a chocolate colour. Sometimes, as with Muskoka Brewery's Double Chocolate Cranberry Stout, Young's Double Chocolate Stout, and Rogue Brewery's Chocolate Stout, the beers are also brewed with a small amount of chocolate or chocolate flavouring.

Imperial stout, also known as "Russian Imperial stout", is a strong dark beer in the style that was brewed in the 18th century by Thrale's Anchor Brewery in London for export to the court of Catherine II of Russia. In 1781 the brewery changed hands and the beer became known as "Barclay Perkins Imperial Brown Stout". It was shipped to Russia by Albert Le Coq who was awarded a Russian royal warrant which entitled him to use the name "Imperial". A recipe from 1856 shows it had an original gravity of 1.107 (almost certainly over 10% abv) and over 10 pounds of hops to the barrel. When Barclay's brewery was taken over by Courage in 1955, the beer was renamed "Courage Imperial Russian Stout" and it was brewed sporadically until 1993.

Imperial stouts have a high alcohol content, usually over 9% abv, and are among the darkest available beer styles. Samuel Smith's brewed a version for export to the United States in the early 1980s, and today imperial stout is among the most popular beer styles with U.S. craft brewers. American interpretations of the style often include ingredients such as vanilla beans, chili powder, maple syrup, coffee, and marshmallows. Many are aged in bourbon barrels to add additional layers of flavour. The word "imperial" is now commonly added to other beer styles to denote a stronger version, hence imperial IPAs, imperial pilsners etc.

Baltic porter is a version of imperial stout which originated in the Baltic region in the 19th century. Imperial stouts imported from Britain were recreated locally using local ingredients and brewing traditions.

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