Black and tan
The name is an allusion to the Black and Tans, soldiers sent to Ireland by the British government in the early 1920s to suppress Sinn Féin revolutionaries agitating for Irish independence.
The most common Black and Tan in the United States uses Guinness Stout and Bass Ale. The Guinness (or other stout) is "layered" on top of the Bass (or other lighter-colored beer), taking advantage of their differing densities. This is accomplished by first halfway filling the glass with Bass, then adding Guinness. The Guinness is best poured slowly over an upside-down tablespoon placed over the glass; this disperses the stout beer and helps to create the desired layering effect in the finished drink.
Combining Guinness and Harp lager (instead of Bass) results in a "Half and half." A "black and white" is a stout with any lager or ale.
A similar beverage using Champagne as one of the mixers is the black velvet, which consists of half champagne and half Guinness. A "poor man's black velvet" subsitutes hard apple cider for the champagne. This variation is also called a "snakebite" by some.
Despite the use of what is arguably Ireland's national beer, the drink is relatively unknown in Ireland and an attempt to order it in a pub there would likely result in either teasing for adulterating the Guinness, or hostility due to the historical relevance of the name.
see also:list of cocktails