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Black and Tan is a drink made from a blend of pale ale and a dark beer such as a stout or porter, most often Guinness. Sometimes a pale lager is used instead of ale; this is usually called a half and half. Contrary to common North American belief, however, Black and Tan as a mixture of two beers is not a term commonly used in Ireland due to the association with the Royal Irish Constabulary Reserve Force, nicknamed the Black and Tans.
The style is believed to have originated in pubs in Britain with drinkers ordering a mix of dark stout and draught bitter. The earliest recorded usage of the term in the Oxford English Dictionary is from 1889, though an earlier origin of an 18th century blend of porter and pale ale has been conjectured. Several American breweries currently make premixed Black and Tan, and it is a popular blend at American bars. One of the oldest and best known commercial examples is Yuengling's Original Black and Tan.
The name "black and tan" had earlier been applied to dogs, such as the black and tan coon-hound. It was later used as a nickname for the Black and Tans paramilitary reserve during the Irish War of Independence.
The most common type of Black and Tan in the United States uses Guinness Draught (not Extra Stout) and Bass, though variations using Harp Lager instead of Bass are referred to as Half and Half. Guinness and Newcastle is generally called a Black Castle. The "layering" of Guinness on top of the ale or lager is possible because the relative density of the Guinness is less than that of the ale or lager. The opposite scenario (where the layer on top is heavier than bottom) would produce the fluid mechanics phenomenon known as the Rayleigh-Taylor Instability.
To prepare a Black and Tan in the American way, first fill a glass halfway with the ale, then add the Guinness Draught (from the can, bottle, or tap). The top layer is best poured slowly over an upside-down tablespoon placed over the glass to avoid splashing and mixing the layers. A specially designed black-and-tan spoon is bent in the middle so that it can balance on the edge of the pint-glass for easier pouring.
In the United Kingdom, another way of preparing a Black and Tan is to pour half a pint of dark stout into a pint glass and then top up with draught bitter, so that both beers are thoroughly mixed together.
In the Republic of Ireland a Black and Tan is normally made from a half pint of Smithwick's topped off with Guinness. This version is also sometimes referred to as a "Pint Special" "Blacksmith" or a "Light and Bitter". During the summer months stout drinkers may order a black and tan due to its lighter texture. Likewise ale drinkers may order a Smithwick's with a Guinness head. This is an ordinary pint of Smithwick's with the last inch or so topped off with Guinness.
In Australia, specifically New South Wales, a Black and Tan is made from half a schooner (425ml) of Tooheys New (a pale lager) and then topped up with Tooheys Old (a dark ale). In South Australia Cooper's Best Extra Stout and Cooper's Original Pale Ale are used.
Contrary to popular belief, the name "Black and Tan" is not used in Ireland as a term for a mixture of two beers. Indeed, the drink has image problems in Ireland and elsewhere due to the association with the Royal Irish Constabulary Reserve Force, nicknamed the "Black and Tans", which was sent into Ireland in the early 1920s. As a result, in Ireland the name is seen as contentious and disrespectful due to the atrocities committed by these troops. 
In March 2006, Ben and Jerry's released an ice cream flavor in the United States for Saint Patrick's Day inspired by the drink; the name offended Irish nationalists because of the paramilitary association. Ben and Jerry's has since apologized. A spokesman told Reuters, "Any reference on our part to the British Army unit was absolutely unintentional and no ill-will was ever intended." In March 2012, the drink's name once more came into the news when Nike, as part of an Irish themed set of designs, released a pair of shoes advertised as the "Black and Tan" and generating offense similar to the earlier Ben and Jerry's ice cream.